When an email campaign does not produce the expected results, SMTP error codes can often provide very useful answers. Indeed, these codes generated during the sending or delivery of emails contain key information about the origin of a problem. Knowing how to decipher them can allow you to turn a failing campaign into an opportunity for improvement. To help you with this decryption, we explain what lies behind the SMTP error codes: how to interpret, react, and take the necessary measures to optimize the deliverability of your messages.
SMTP error code: what’s it all about?
An SMTP error code is a response sent by the recipient’s email server to your sending server. In general terms, we talk about SMTP response codes. These indicate whether the email has been successfully delivered or whether an error has occurred. In the latter case, it is therefore an SMTP error code.
Structure of an SMTP error code
The first digit indicates the general nature of the status (temporary or permanent error).
The second digit specifies the category of the error (for example, error related to the email system or address).
The third digit provides additional details on the exact cause of the error.
Categories of SMTP error codes
1) 4xx codes: Temporary errors
They signal temporary issues that can be automatically resolved by retrying.
421 Service not available – The server is temporarily unavailable, often due to overload or maintenance. In this case, the sending system will generally try to resend the email later.
450 Requested action not taken – The recipient’s email address is temporarily inaccessible. This can be due to a full inbox or a temporary server-side issue.
451 Action aborted – An internal server error prevents delivery. This code may also appear if the email content is suspected of containing problematic elements or if the server is experiencing a temporary overload.
2) 5xx codes: Permanent errors
They indicate definitive issues that prevent message delivery.
530 Authentication required: This code reflects an authentication error. The sender must check their SMTP settings and ensure authentication is enabled.
550 Requested action not taken – The email is refused because the address does not exist or is not valid. This can also be due to DNS configuration issues or a blacklist.
551 User not local: The server indicates the user is not hosted locally and no forwarding is set. It may require contacting the recipient to resolve the issue.
552 Requested mail action aborted: exceeded storage allocation: This code means the recipient’s mailbox is full and cannot receive any more messages. Check the validity of the addresses and plan a retry later.
553 Requested action not taken: mailbox name not allowed: The server rejects the email address due to incorrect syntax or invalid format. It may also indicate the address is on a blacklist.
554 Transaction failed: The email has been rejected by a strict filtering policy, often related to spam suspicion. This code may also indicate your domain or IP is blacklisted.
How to manage and resolve SMTP error codes
Maintain a clean mailing list
- Remove invalid addresses and update your lists in real-time. Use validation tools to detect inactive or incorrect addresses before sending.
- Segment your contacts according to specific criteria (interest, engagement history, location) to better target your campaigns and avoid unnecessary bulk sending.
- Implement a double opt-in process to ensure the addresses added to your lists are valid and consenting, reducing bounce risks.
- Identify contacts who have not interacted with your emails for some time and consider removing them or sending a re-engagement campaign.
Monitor the reputation of the IP address and domain
- Use monitoring tools to track the status of your domain and IP addresses. These provide detailed reports on your sender reputation, as well as alerts in case of issues.
- Authenticate your emails with protocols such as SPF, DKIM, and DMARC to enhance the credibility of your messages and reduce the risk of rejections.
- Regularly check that your domain or IP addresses are not listed on blacklists. Take steps to quickly resolve any issues.
Test and optimize your campaigns
- Analyze your campaigns to eliminate any elements that may trigger spam filters, such as risky keywords, excessive special characters, or poorly integrated images.
- Test different versions of your emails (subject, content, design) to identify the elements that generate the most engagement and maximize your open rate.
- Go beyond simply adding names in the subject; also personalize the content based on your recipients’ preferences or history.
Although they may not be pleasant to receive at first, remember that SMTP error codes are valuable allies that alert you to issues with an email campaign. Therefore, it is crucial to know how to interpret and manage them correctly.
