15 Most Common SMTP Error Codes and How to Fix Them Step by Step

Have you ever wondered why some of your emails never reach their destination? Approximately 16% of all emails disappear along the way due to errors. The SMTP protocol acts as the digital postman that determines whether your messages arrive correctly or are lost in the process.

In this article, you’ll learn how to identify and resolve the most common SMTP error codes you encounter when sending emails. With more than 6 million emails handled daily, these failures are a daily reality for millions of users.

SMTP error codes work as a medical diagnostic system for your emails. These three-digit numbers tell you exactly why your message failed to deliver. The first digit reveals whether it is a success, a temporary problem, or a definite error, while the next two offer specific details about the cause.

Remember that there is a fundamental difference between these codes: temporary errors (4xx) allow you to try the send again, but permanent errors (5xx) indicate a definitive rejection. When you don’t understand these codes, your email marketing campaigns and important communications can be severely affected.

Mastering these codes isn’t optional if you want to maintain effective email communication. We’ll show you the 15 most common SMTP error codes you’ll encounter in 2025, explaining each one clearly and providing you with practical solutions to resolve them quickly.

Keep in mind that you can always automate the process of detecting invalid emails by using the Verify Email Address Validation API .

Key Points

SMTP error codes are critical for diagnosing email delivery issues and maintaining effective communications:

• 4xx codes indicate temporary errors that are resolved by retrying, while 5xx codes are permanent and require specific action

• 16% of emails do not arrive due to delivery errors, with SMTP codes being key to identifying and fixing these issues quickly

• Common errors such as 550 (mailbox non-existent) and 552 (quota exceeded) have straightforward solutions: verify addresses and reduce sizes

• Correct configuration of SMTP authentication and DNS records prevents most 503, 553, and 554 errors

• Implementing preventive checks and using the reference table significantly improves the effectiveness of your email communications

Mastering these codes will allow you to solve email problems efficiently, avoiding major communication losses and optimizing your email marketing campaigns.

SMTP Error 421 – Service Unavailable

SMTP protocol error 421 appears as a temporary obstacle when you try to send emails. Unlike other definitive errors, this problem usually resolves automatically over time.

Meaning of Code 421

This code tells you that the SMTP service is temporarily unavailable. In layman’s terms, the server is telling you that at that specific time it can’t process your submission request. On the other hand, codes that begin with 4 (such as 421) indicate temporary failures, while those that begin with 5 indicate permanent errors.

Common Causes of SMTP Error 421

Here are the main reasons why you see this error:

• Connectivity issues between your server and SMTP server
• SMTP server overloaded or temporarily out of service
• Security or firewall settings blocking the connection
• Restarting the mail service or maintaining the server
• DNS resolution issues preventing connection
• Sending limits exceeded at your service provider
• Temporary failure in the line of communication

Statistics show that approximately 16.9% of all emails do not reach their recipients due to delivery errors, with error 421 being one of the significant contributors.

How to Fix SMTP Error 421

Fortunately, you can resolve this issue by following these steps:

Step 1: Wait and retry the send – Since this is a temporary error, waiting a few minutes and trying again usually fixes the problem.

Step 2: Check your internet connection and check the stability of your network.

Step 3: Check the status of the SMTP server to confirm that it is operational.

Step 4: Review the firewall settings to make sure it doesn’t block SMTP traffic.

Step 5: Confirm your DNS settings to verify that they are correctly configured.

Step 6: Change the connection port – If you’re using port 25, try port 587 along with STARTTLS for added security.

Step 7: Verify your SMTP authentication credentials to confirm that they are correct.

Step 8: Spread your mailings over time instead of sending mass mailings all at once.

Mastering the operation of the SMTP protocol will allow you to deal with this common error and keep your communications flowing without interruptions.

SMTP Error 450 – Mailbox Unavailable

The SMTP 450 error appears when you try to send an email and the server responds with a “not now, try again later”. Unlike a final rejection, this code points to a temporary hurdle that you can overcome with the right strategies.

Meaning of Code 450

This code informs you that the mail server can’t complete your request because the recipient’s mailbox isn’t available at the moment. Think of it like when you call on the phone and the line is busy: it doesn’t mean that the number doesn’t exist, but that you should try it later.

Error 450 belongs to the family of 4xx codes, which indicate temporary problems in mail delivery, as opposed to 5xx codes that represent permanent errors. This difference is critical to determining your next step.

Common Causes of SMTP Error 450

Several situations can cause this error:

• Busy or saturated mailbox: The recipient may be receiving another email or have exceeded their storage quota
• Incorrect DNS configuration: Domain configuration errors or DNS resolution issues
• Active greylisting system: Anti-spam technique that temporarily rejects emails from unknown senders
• Overloaded Server: High traffic on the receiving server
• Restrictive Policies: Target Server-Specific Limitations
• Connectivity issues: Difficulties in communication between servers

How to Fix SMTP Error 450

To resolve this issue, we recommend following these steps:

Step 1: Please wait 15-30 minutes before retrying the send, as this is a temporary error

Step 2: Verify that you have typed the recipient’s address correctly, paying special attention to the domain

Step 3: Check if the DNS of the target domain resolves correctly using tools such as nslookup

Step 4: If you specifically receive error 450 4.7.1, check your sender reputation as it could indicate issues with your IP

Step 5: Consider using an alternate SMTP server if the problem persists

Step 6: Contact your provider’s support if none of the above solutions work

By understanding the temporary nature of this error, you will be able to effectively handle these interruptions in your email communications.

SMTP Error 451 – Local Error on Server

Code 451 represents one of the most common temporary errors you’ll encounter when using the SMTP protocol. Unlike permanent failures, this problem is usually resolved without manual intervention, although knowing its causes will help you take action when necessary.

Meaning of Code 451

When you receive an SMTP 451 error, the server is letting you know that it has encountered a temporary problem that is preventing it from processing your email request. Typical messages include “451 Temporary Local Problem” or “Requested action aborted: local error in processing”.

You’ll come across specific variants like “451 4.3.2,” which indicates that the server can’t process commands because your system isn’t accepting messages at the time. There is also the “451 4.4.2 timeout”, which indicates that the connection was closed before it was completed.

Common Causes of SMTP Error 451

The main reasons for this error include:

• Server overload: The server receives more requests than it can handle and rejects new requests
• Scheduled maintenance: Servers in the process of being upgraded or experiencing technical difficulties
• Resource limitations: Issues with CPU, memory, or available bandwidth
• Unstable connections: Network issues that cause excessive wait times
• Poor DNS Configuration: Incomplete or Incorrect DNS Records
• Anti-spam filters: Temporary restrictions when suspicious activity is detected
• Ancillary services: Bugs in tools such as SpamAssassin or ClamAV

How to Fix SMTP Error 451

Here are the most effective actions to solve this problem:

Step 1: Implement the waiting
strategyBecause this is a temporary error, waiting 10-30 minutes before retrying the send usually resolves the situation.

Step 2: Check the status of the service
Check your SMTP provider’s status page or contact their support to identify known issues.

Step 3: Review your DNS
settingsMake sure you’ve added all the DNS records provided by your email provider.

Step 4: Set up an alternate
serverIf the problem persists, temporarily switching to a backup SMTP server can keep your shipments running.

Step 5: Automate retries
Set up your system to automatically retry failed submissions after specific intervals.

Step 6: Examine your firewall
settingsTemporarily disable restrictions that might be blocking SMTP connections.

If you have any questions about resolving this error, our support team will be happy to help you optimize your email settings.

SMTP Error 452 – Insufficient Storage

Receiving the message “452 Requested action not taken: insufficient system storage” when sending an email indicates a direct problem with storage capacity. This specific code requires immediate actions to restore the flow of your communications.

Meaning of Code 452

The SMTP code 452 communicates that the server is unable to complete your request due to lack of available storage space. This error focuses exclusively on capacity issues: the recipient’s mailbox reached its maximum limit or the server exceeded its full capacity. Unlike other SMTP errors that can be temporarily ignored, this one permanently blocks delivery until the space problem is resolved.

Common Causes of SMTP Error 452

Situations that cause this error include:

• Recipient mailbox overcrowded: Account has reached maximum storage limit allowed
• Space-constrained server: The entire server experiences restrictions affecting multiple users
• Restrictive Server Configuration: Administrators reduced size thresholds to optimize resources
• Concentrated bulk sends: Too many messages in short periods trigger protective measures
• Large messages: Emails that exceed the available capacity in the destination account

How to Fix SMTP Error 452

We recommend that you implement these specific solutions:

Step 1: Optimize your shipments

• Spread shipments over longer periods
• Set hourly limits using control tools
• Compress images and reduce attachments
• Simplifies HTML code to minimize the space required
• Segment large lists into manageable groups

Step 2: Manage mailbox full cases

• Contact the recipient by alternative means to request a space release[181]
• Provides instructions on mailbox management (archiving, deleting old emails)

Understanding these limitations of the SMTP protocol allows you to anticipate storage issues and keep your communications running efficiently.

SMTP Error 500 – Syntax Error

The 500 code represents one of the most technical errors of the SMTP protocol, as it indicates a fundamental failure in the communication between your email client and the destination server. This error requires a specific approach to resolve it correctly.

Meaning of code 500

The SMTP 500 error appears when the server can’t interpret the SMTP commands you send to it. Specifically, it indicates that the server has been unable to recognize the command due to an error in the way it is written or structured. Your email client is sending instructions that the server can’t process.

Common Causes of SMTP 500 Error

This problem arises for several technical reasons:

• Conflict between the server and your system’s antivirus software
• SMTP commands that are too long or malformed
• Incorrectly configured firewall or antivirus rules on the target server
• Communication problems between your email client and the server
• Incorrect configuration in SMTP connection parameters

How to Fix SMTP Error 500

We recommend that you follow these actions to resolve the issue:

Step 1: Check your antivirus settings – Temporarily disable protection to check if it’s interfering with SMTP commands

Step 2: Check the firewall rules – Check that they are not blocking SMTP communication between your client and the server

Step 3: Use alternate ports – If the error appears with the message “Access Denied By Port Access”, try other standard SMTP ports (587 or 465) instead of the default (25)

Step 4: Simplify your messages – Reduce the size and complexity if you get the “Line too long” error

Step 5: Update your email client – Make sure you are using an updated version that complies with the current standards of the SMTP protocol

Remember that the operation of the SMTP protocol is based on precise communication between client and server. Understanding these technical aspects will allow you to maintain more reliable email communication.

SMTP Error 501 – Incorrect Parameters

Seeing a “501 Syntax error in parameters or arguments” message when sending emails tells you that there is a specific formatting issue that you need to fix in order to establish communication with the SMTP server.

Meaning of the 501 code

501 code points out syntax errors in the parameters or arguments of SMTP commands. While error 500 points to problems with the full command, this error focuses only on the parameters that accompany the command. The server recognizes your sending attempt, but detects some incorrect formatting in the data provided.

Common Causes of SMTP 501 Error

You’ll run into this error for these main reasons:

• Poorly formatted email addresses
• Prohibited special characters (quotation marks, accents, rare symbols)
• Unnecessary spaces in addresses
• Copying and pasting errors that include hidden characters
• Interference of antivirus software with communication

How to Fix SMTP Error 501

We recommend that you follow these steps to resolve the issue:

1. Thoroughly check each address: Verify that all email addresses comply with the [email protected] format
2. Remove problematic characters: Delete quotation marks, extra spaces, and special characters from addresses
3. Temporarily disable antivirus: Check if your security software is interfering with SMTP communication
4. Type addresses manually: Avoid copying and pasting addresses that may contain hidden characters
5. Check your Outlook settings: If you’re using this client, make sure there are no typos in the recipients

Understanding these details of the SMTP protocol will allow you to avoid syntax errors and maintain smooth email communication.

SMTP Error 502 – Command Not Implemented

 

Have you tried to use a feature in your email client and got a 502 error? This code indicates that the server can’t execute what you’re asking of it because it simply doesn’t have that capacity available.

Meaning of the 502 code

The SMTP 502 error is telling you that the SMTP command you’re trying to use doesn’t exist on the email server. It’s like asking an old TV to play streaming content: specific functionality is either not implemented or disabled in the server’s SMTP connector.

Common Causes of SMTP 502 Error

This error appears for specific reasons that you should be aware of:

• The feature you need isn’t programmed into the server
• The command is disabled by server security policies
• You use advanced features on a server with basic configuration
• The administrator has disabled certain options for security reasons

How to Fix SMTP Error 502

To resolve this obstacle, we recommend following these steps:

1. Contact your service provider: Ask them specifically about the availability of the command you need to use
2. Check your server specifications: Check if your current plan includes the feature you’re trying to use
3. Look for alternatives: Ask for guidance on other methods to achieve the same result
4. Consider switching providers: If functionality is essential to your needs, consider other SMTP services that offer it

Each SMTP server has its own limitations and capabilities. Understanding these differences will help you better manage your email communications and avoid unnecessary frustration.

SMTP Error 504 – Parameter Not Implemented

The 504 code represents one of the most technical errors in the SMTP protocol. You will run into this obstacle when the specific parameters of the commands are not available on the target server.

Meaning of the 504 code

The server is communicating to you that it recognizes your command, but it can’t process it because it contains parameters that aren’t implemented in its current configuration. It’s like trying to use an advanced feature on a device that only has basic capabilities – the device understands what you’re asking for, but just can’t run it.

In addition, this error arises when parameters are lost during transmission or when the provider’s server has not implemented certain specific functionalities.

Common Causes of SMTP 504 Error

We recommend you review these aspects that frequently cause this error:

• Incorrect authentication settings, especially when the server requires pre-send authentication
• Deprecated authentication mechanisms such as XOAUTH, which Google no longer supports (504 error 5.7.4)
• SSL/TLS configurations that cause excessive timeouts
• Overly large attachments sent over SSL connections
• Incorrect sequence in sending commands to the server

How to Fix SMTP Error 504

To resolve this technical issue, follow these steps:

1. Review your SMTP authentication settings and check the correct options in your email client
2. Verify that you meet all security requirements set by the server
3. Replaces outdated authentication mechanisms with updated versions
4. If you send attachments, reduce their size to avoid transmission issues
5. Contact your provider to find out exactly what parameters their server supports

Understanding how the SMTP protocol works and its technical limitations will allow you to overcome this error and keep your communications working properly.

SMTP Error 550 – Mailbox Does Not Exist

The SMTP 550 error represents one of the most definite obstacles you will encounter when sending emails. Unlike the temporary errors we’ve seen above, this code signals a permanent rejection that won’t resolve on its own.

Meaning of Code 550

When you receive this code, the destination server is communicating a resounding “no” to your email. The standard message appears as “550 Requested action not taken: mailbox unavailable.” Essentially, the server can’t deliver your message because the recipient’s address doesn’t exist or because you’ve been blocked by specific server policies.

Common Causes of SMTP Error 550

We recommend you review these factors that cause this error:

• Misspelled or typo email address
• Recently deactivated or deleted email account
• Non-existent or no longer operating domain
• Rejection for server policy violations
• Lack of successful SMTP authentication
• Problems with the sending domain’s SPF records
• Specific blocking of your address or domain

How to Fix SMTP Error 550

To resolve this issue for good, follow these steps:

Step 1: Carefully check the recipient’s address, looking for typos or incorrect punctuation marks

Step 2: Contact the recipient by another means to confirm their current address

Step 3: Properly configure SPF, DKIM, and DMARC records to improve the authenticity of your emails

Step 4: Enable SMTP authentication in your email client by checking the “My server requires authentication” option

Step 5: Check that your IP is not blacklisted using tools such as talosintelligence.com

Remember that the operation of the SMTP protocol requires valid addresses and correct configurations to establish successful communication.

SMTP Error 551 – Non-Local User

Have you received the message “551 User not local” and don’t know what it means exactly? This SMTP protocol error indicates a specific redirection issue that is blocking your mail delivery through the usual route.

Meaning of Code 551

Code 551 informs you that the recipient is not hosted locally on the server that received your message. In simple terms, it’s like arriving at a post office and being told that the person you’re looking for doesn’t live in that city. The server detects that the address does not belong to your domain and may occasionally suggest an alternate address for forwarding. Basically, the domain exists but doesn’t manage mailboxes for that specific address.

Common Causes of SMTP Error 551

Why does this error appear? The main reasons include:

• The recipient’s mailbox does not exist on the server that processed your request
• A proper redirect has not been set up for that user
• The server uses this mechanism to prevent SPAM by blocking SMTP Relay
• The destination domain operates but does not directly handle that specific mailbox

How to Fix SMTP Error 551

We recommend that you be realistic about the options available. Unlike other SMTP errors, this issue is not dependent on your settings, which significantly limits the solutions:

1. Contact the recipient directly to confirm if their email address is correct
2. Enable SMTP Relay if you manage the server and need to allow forwarding
3. Check for possible typos in the email address
4. Check for provider changes – the domain could have migrated to another email service

If you have any questions about this specific error, our team will be happy to help you diagnose the particular problem with your email communication.

SMTP Error 552 – Storage Quota Exceeded

When your email bounces with the message “552 Requested action not taken: insufficient system storage”, you’re facing one of the most frustrating problems with sending emails. This code indicates that you have hit a space barrier that prevents the delivery of your message.

Meaning of Code 552

This code indicates that the send action has been canceled because the allocated storage capacity has been exceeded. In essence, it communicates that the recipient has exceeded their quota of available space and cannot receive any more messages. In addition, this error can also appear when you try to send an attachment that exceeds the maximum size allowed by the remote server.

Common Causes of SMTP Error 552

Several factors can trigger this problem:

• The recipient’s mailbox is completely full
• The size of the message with attachments exceeds the server limits (usually 25MB)
• The attachment is of an unauthorized type
• The number of attachments exceeds the set limit
• Header values are too large

How to Fix SMTP Error 552

We recommend that you implement these effective solutions:

1. Examine the total size of the message including attachments
2. Compress files or use external services such as WeTransfer
3. Contact the recipient to free up space in their mailbox
4. Upload the file to your website and share the URL instead of the attachment
5. Split large files into several smaller emails

Intelligent email size management significantly improves the deliverability of your email marketing campaigns. If you’re going to be sending heavy content on a regular basis, consider setting up automatic limits on your system to avoid this recurring error.

SMTP Error 553 – Address Not Allowed

If you see a 553 error message during the sending of emails, you are facing an authorization problem that blocks communication with the receiving server.

Meaning of Code 553

The destination mail server rejects your message because it considers that the sender’s address does not comply with its acceptance policies. This code specifically points to a problem with the address used to send the mail or with the formatting of the content. When you see the message “553 sorry, that domain isn’t in my list of allowed rcpthosts,” it means that your domain isn’t authorized to send emails to that server.

Common Causes of SMTP Error 553

This rejection can originate for several reasons:

• Missing PTR record: The destination server blocks messages from servers that do not have the reverse resolution DNS record configured correctly
• Incorrect address formatting: Addresses with formatting errors or special characters such as eñdas or accents
• SMTP authentication disabled: You haven’t checked the “My server requires authentication” box
• Invalid content: The format of the mail content does not comply with the standards of the destination server
• Domain configuration issues: Poor configuration preventing proper validation

How to Fix SMTP Error 553

To overcome this hurdle, we recommend following these steps:

1. Enable the “My server requires authentication” option in your email client settings
2. Verify that the sender’s address is properly formatted and valid
3. Properly configure the PTR record for your mail server
4. Simplify the formatting of your messages, avoiding complex HTML code when it is not needed
5. Review SMTP server settings to ensure authorized delivery

Mastering how the SMTP protocol works will allow you to resolve these types of authorization errors efficiently.

SMTP Error 554 – Transaction Failed

Have you received a 554 error when sending emails? This code represents one of the most serious blocks of the SMTP protocol, indicating that your message has been definitively rejected without the possibility of automatic retry.

Meaning of code 554

The SMTP 554 error communicates that the entire transaction between your server and the recipient’s server has failed. Unlike the temporary errors we have seen above, this code signals a permanent and definitive rejection. When the “554 5.7.1” variant appears, it specifically means that your IP address is blocked or that the content of your message has triggered spam filters.

Common Causes of SMTP Error 554

The main reasons that trigger this crash include:

• Your IP is blacklisted: Mail servers have flagged your IP address as a source of spam
• Content detected as spam: Keywords or message structure trigger automatic filters
• Invalid email addresses: Recipients do not exist or contain typing errors
• SMTP authentication disabled: You haven’t enabled “My server requires authentication”
• Incomplete DNS records: Missing essential settings from your email provider

How to Fix SMTP Error 554

We recommend you follow this step-by-step solution protocol:

Step 1: Reputation
DiagnosisUse tools like MultiRBL or MXToolBox to check if your IP is blacklisted. This verification is critical before any further action.

Step 2: Debug your content
Remove words that trigger spam filters such as “Congratulations,” “Shop Now!” or “Special Offer. It simplifies HTML and avoids excess images.

Step 3: Authentication
settingsEnable the “Server requires authentication” option in your email client. This configuration is mandatory for most modern servers.

Step 4: Implement Security
ProtocolsProperly configure SPF, DKIM, and DMARC to establish the legitimacy of your shipments. These DNS records are essential to your reputation as a sender.

Step 5: Update credentials
Regularly change your passwords to prevent unauthorized access that can damage your reputation.

If you need help with the implementation of these solutions, our support team will be happy to guide you through the process of recovering your shipping reputation.

SMTP Error 503 – Incorrect Script

When your mail client “talks” to the SMTP server in the wrong order, the 503 error appears. This code is specifically triggered when SMTP commands do not follow the proper sequence during communication.

Meaning of the 503 code

The 503 code informs you that the server has detected commands sent out of order or that you need to authenticate first. Basically, it’s like trying to open a door before inserting the key: the steps should follow a specific order. The typical message “503 Valid RCPT command must precede DATA” means that you must specify the recipient before sending the message content.

Common Causes of SMTP 503 Error

This error appears mainly for these reasons:

• Your server requires authentication but you haven’t enabled it
• You try to send data before you set who the recipient will be
• The connection is interrupted and alters the natural order of the commands
• Incorrect settings on your mail server
• You haven’t checked your mailbox before sending

How to Fix SMTP Error 503

To resolve this issue, follow these steps:

1. Go to your email client’s settings and check “My outbound server (SMTP) requires authentication”
2. Confirm that you have the correct server address, the right port, and the appropriate authentication method
3. If you program your own scripts, check that the commands are sorted correctly
4. Disable your VPN temporarily, as it may interfere with authentication
5. Check your mailbox before attempting new mailings

Quick Comparison Table

Need a quick reference to identify SMTP error codes? This table will allow you to diagnose problems immediately and apply the correct solution without wasting time.

Remember that knowing these codes will save you hours of frustration when managing your email marketing campaigns. Use it as a quick reference guide when you encounter any of these errors:

Error Code	Type of Error	Main Meaning	Main Causes	Main Solution
SMTP 421	Service not available	SMTP service temporarily unavailable	Connectivity issues, overloaded	Wait and retry the shipment
SMTP 450	Mailbox not available	Recipient’s mailbox temporarily inaccessible	Mailbox busy, greylisting, server	Wait and retry after a few minutes
SMTP 451	Local Server	Temporary server	Server overload, maintenance, resource	Wait and retry later
SMTP 452	Insufficient	Not enough storage	Recipient mailbox full, server limits exceeded	Reduce message size or free up space
SMTP 500	Syntax	Command not recognized by the server	Conflicts with antivirus, malformed commands	Review antivirus settings
SMTP 501	Incorrect	Command	Incorrect formatting in addresses, special	Review email address formatting
SMTP 502	Command not implemented	Feature not available on the server	Feature not deployed or disabled	Contact Service Provider
SMTP 503	Incorrect	Incorrect SMTP	Authentication not enabled, wrong	Enable SMTP authentication
SMTP 504	Parameter not implemented	Parameter not supported by the server	Incorrect authentication	Verify authentication settings
SMTP 550	Mailbox does not exist	Non-existent	Misspelled address, deactivated	Verify Recipient Address
SMTP 551	Non-local	User not hosted on the current	Mailbox not available locally	Contact the recipient to confirm address
SMTP 552	Quota exceeded	Exceeding	Mailbox full, message too large	Compress files or use external services
SMTP 553	Address Not Allowed	Server rejects sender	Missing PTR registration, incorrect	Configure DNS records correctly
SMTP 554	Failed	Final rejection of the transaction	Blacklisted IPs, suspicious	Check IP status on blacklists

Save this table as a reference to quickly resolve any issues you encounter in your email sends.

Conclusion

You’ve learned how to identify and resolve the 15 most common SMTP error codes you’ll encounter when managing your emails. Each code provides you with specific information about the problem, allowing you to act efficiently to fix it.

The difference between temporary and permanent codes sets the tone for your resolution strategy. 4xx errors require patience and retries, while 5xx errors require specific actions such as verifying addresses or adjusting authentication settings.

When you understand that a 550 error indicates a non-existent address or a 452 indicates storage issues, you can resolve these obstacles quickly without wasting time on unnecessary testing. This direct understanding significantly improves the effectiveness of your communications.

We recommend keeping the comparison table as a quick reference during your email sending. This tool will save you valuable time by diagnosing problems immediately.

To optimize your future communications, consider implementing preventive checks on your systems. Validating email addresses before sending, correctly configuring DNS records, and maintaining clean lists are practices that significantly reduce the occurrence of these errors.

Remember that mastering these codes not only solves technical problems, but also protects your reputation as a sender and maximizes the return on investment of your email marketing campaigns. Each correctly resolved error represents one more successful communication with your important contacts.

 

FAQs

Q1. What do SMTP error codes mean?
SMTP error codes are three-digit numbers that indicate the delivery status of an email. The first digit indicates whether it is a success, temporary or permanent error, while the other two provide more specific information about the problem.

Q2. Why do I get an error when sending an email?
An error when sending an email usually indicates that the message could not be delivered to the recipient. This can be caused by SMTP server issues, incorrect credentials, or security configuration lockouts.

Q3. What is the SMTP protocol?
SMTP (Simple Mail Transfer Protocol) is a communication protocol used to send and receive e-mail messages over the Internet. Mail servers and other message transfer agents use it to manage email traffic.

Q4. How do I end an SMTP session?
To end an SMTP session, you use the QUIT command. This command requests session termination, and the server must respond with code 221 OK before the client closes the connection.

Q5. What is the percentage of emails that don’t reach their destination due to errors?
Approximately 16% of all emails never reach their destination due to various errors in the delivery process. This underscores the importance of understanding and resolving SMTP error codes to improve the effectiveness of email communications.