SMTP servers are crucial for seamless email delivery. However, sometimes they encounter errors and almost every error has a specific code. Knowing about those error codes and learning how to fix them is important for smoother operations.
In this post, we are going to discuss about some of the most common SMTP errors, including how you can identify those and fix those before they make a significant impact on your email marketing efforts.
Well, before we go ahead, let us have a quick look at what SMTP errors are.
What Do You Mean by SMTP Errors?
SMTP errors are fault codes that servers issue in response to failure email delivery or processing. There are different types of error codes and each one has a specific numerical code and message. By looking at the error code number, one can identify the type of error that has occurred.
Now, to troubleshoot these issue in email delivery, you need to understand these codes. We’ll talk about this below. But, let us have a quick look at what is the reason behind SMTP errors.
Why SMTP Errors Occur?
Some of the common reasons behind the occurrence of SMTP errors are:
- Incorrect login or authentication details.
- Blocked ports or firewall restrictions.
- Syntax errors.
- Temporary problems or server overloads.
- Restrictions at the recipient’s end or exceeding storage limits.
The best way to prevent such errors is to proactively manage server settings and keeping authentication details up-to-date.
Common SMTP Errors and How to Fix Them?
Some of the most common SMTP errors that you may encounter include:
1. Error 421 – Service Not Available
SMTP error 421 is a temporary error which indicates that server is unavailable. The reason for this could be server overload or restrictions in firewall settings.
How to Fix This?
Wait for sometime and retry later.
Make sure that SMTP server provider is operational.
Check firewall and port settings.
3. Error 451 – Server Error
If you have any server error, you will see SMTP code 451. The reason behind such an error could be overload, capacity problems, or spam filtering policies.
How to Fix This?
Reconsider email sending frequency. Reduce it.
Check the restrictions on email service provider side.
Avoid sending bulk emails on a quick note.
3. Error 452 – Too Many Recipients
This server error message appears when you have more number of recipients in your email list than the server’s limit.
How to Fix This?
Reduce the recipient count from the list.
Split emails into small batches.
If your list is growing, upgrade server capacity.
4. 500/501/502/503/504 – Syntax and Command Errors
These error codes typically reflect problems with SMTP command formatting, unsupported commands, and wrong command sequencing.
How to Fix This?
Double-check command syntax and options.
Review sender and receiver configuration.
Ensure proper authentication sequence before sending mail.
5. Error 550 – Relaying Denied / Requested Action Not Taken
This SMTP error 550 occurs when the server don’t allow sender to relay emails through it. The common reason behind this is that the sender is not authenticated or authorized. The causes for the same may include wrong authentication details, strict security settings, or issues with server configuration.
How to Fix This?
Ensure login authentication is active.
Make sure that server settings allow relaying.
Update security protocols as required.
6. Error 552 – Storage Limit Exceed
This error is because of recipient-side problem. It indicates that the recipient’s inbox is full and cannot accept more messages.
How to Fix This?
Ask the recipient to clear mailbox space.
Wait and resend the email.
Suggest recipient to upgrade the mailbox if problem persists.
7. Error 553 – Invalid Recipient Address
This SMTP error 553 indicates that the email address of the recipient is either incorrect or not available.
How to Fix This?
Double-check the email address formatting.
Remove unnecessary spaces and characters.
Verify domain name accuracy.
8. Error 554 – Transaction Failed
A generic error, 554 signals an unsuccessful transaction. The reasons for this may be incompatible email format, oversized attachments, or server misconfiguration.
How to Fix This?
Reduce attachment size.
Check and standardize email format.
Adjust server settings for compatibility.
9. Authentication Errors: “SMTP Error – Could Not Authenticate”
Occurs mostly due to wrong or expired credentials, mismatched authentication methods, or firewall/Security software interference.
How to Fix This?
Verify username, password, and authentication method.
Review SMTP server, port, and encryption settings.
Temporarily disable firewalls to test for connection issues.
How to Identify SMTP Errors Quickly?
The fastest way to identify SMTP errors is to:
- Read the error code and message from bounce backs or logs.
- Reference the error code in official documentation or troubleshooting guides.
- Use tools like Telnet to test server authentication and connectivity.
- Analyze SMTP server logs for specific error messages, timestamps, and IP addresses.
When troubleshooting, always start by confirming basic settings—server address, port numbers, username/password, and encryption protocols. Often, correcting a simple misconfiguration resolves the problem instantly.
Fast Fixes for SMTP Errors
Here’s a checklist for quick resolution:
- Verify Server Settings: Ensure correct SMTP server address and port (common ports are 25, 465, or 587). Match your encryption (SSL/TLS) to provider’s settings.
- Test Authentication: Double-check login credentials and authentication types (plain, login, OAuth, etc.).
- Check Firewall and Ports: Open essential ports and allow SMTP traffic through firewalls.
- Update Software: Use updated email clients to avoid incompatibility.
- Reduce Attachments & Recipients: Keep attachment size reasonable and limit recipient number per email.
- Retry Later: For temporary errors like 421 or 451, waiting and resending may solve the issue.
Conclusion
Although temporary, SMTP server errors can put a stop on your smooth email campaigns. Hence, it is important for you to know how those errors can be fixed. We have highlighted some of the common STMP errors that people encounter. Along with that, we have also tried to tell some tips to fix those errors.
