In the context of email sending, one of the least visible yet critical aspects is understanding SMTP response codes. These codes, exchanged between servers during message transmission, can provide very useful information for your email campaigns. Knowing how to interpret them is a major asset for optimizing delivery rates and quickly identifying issues. Here, we explain exactly what these response codes are and the main ones you need to know.
What is an SMTP response code?
An SMTP code is a standardized response sent by a mail server to indicate the status of an email. These responses communicate whether the message has been accepted, temporarily delayed, or permanently rejected. They play a crucial role in server-to-server communication as they allow senders and recipients to identify and resolve issues concerning email transmission.
For marketing professionals, SMTP response codes also help improve deliverability and protect their sender reputation because certain codes can indicate a blacklisting.
SMTP codes are composed of three digits: the first indicates the class of the response (success, temporary error, or permanent error), while the other two digits provide more specific information about the exact nature of the response.
Here is a general classification:
- Codes 1xx: Informative responses (rarely used in SMTP).
- Codes 2xx: Success (the email was accepted).
- Codes 3xx: Redirection or further action required.
- Codes 4xx: Temporary errors (the email can be retried).
- Codes 5xx: Permanent errors (the email is permanently rejected).
Understanding the difference between SMTP code, SMTP relay, and SMTP protocol
The SMTP protocol is the general framework governing the sending of emails.
The SMTP relay is a specific application where a server acts as an intermediary.
SMTP codes are specific protocol responses to signal the state of a transmission.
The most common SMTP response codes and their meaning
Success codes (2xx)
They indicate that the message has been successfully received by the destination server.
250 – OK: The message has been accepted and delivered to the recipient.
Meaning: Good news! This means your email has reached its destination.
251 – Recipient not local, message will be forwarded: The server will forward the email to another server.
Meaning: It may indicate a transfer to an external domain, but the message will still be delivered.
252 – Cannot verify recipient’s mailbox: The email address is accepted, but the availability of the user account cannot be immediately verified.
Meaning: This may imply a bounce risk if the mailbox does not actually exist.
Temporary error codes (4xx)
Codes 4xx mean that the server is experiencing a temporary problem. Sending the email can be retried later.
421 – Service unavailable: The server is temporarily unavailable due to a connection issue.
Meaning: Your messages may be delayed. Monitor whether this code persists.
450 – Mailbox unavailable: The account is temporarily inaccessible (for example, the mailbox is full).
Meaning: Encourage your recipients to clean up their mailbox to avoid this issue.
451 – Action aborted: Server error: An internal error prevents the message from being sent.
Meaning: This may result from a problem on the sender or recipient’s server.
452 – Insufficient storage: The server does not have enough space to handle the email.
Meaning: Ensure that your servers and those of your recipients are properly configured.
455 – Server parameter temporarily unavailable: Certain server limitations temporarily prevent reception.
Meaning: This may mean there is a load issue on the server.
Permanent error codes (5xx)
These codes indicate that the email is permanently rejected.
550 – Mailbox not found: The email address does not exist.
Meaning: Regularly check your address lists to keep them clean.
551 – Relay not permitted: The destination server does not allow the relay.
Meaning: Use an authenticated SMTP server to work around this issue.
552 – Quota exceeded: The recipient has exceeded their storage capacity.
Meaning: Communicate with your recipients to increase their storage space.
553 – Invalid domain name: The email address contains an invalid or misconfigured domain.
Meaning: Check the spelling and DNS configuration of the addresses on your list.
554 – Transaction failed: Email sending is rejected.
Meaning: This may indicate a block of your domain. Check your sender reputation.
4 tips for managing SMTP codes
- Actively monitor SMTP responses: Use analysis software or an emailing platform that displays the codes in real-time.
- Maintain a clean address list: Remove invalid addresses to avoid bounces.
- Regularly test your SMTP settings: Ensure that your server is properly configured.
- Resolve issues quickly: Address errors immediately to avoid impacting future campaigns.
If you haven’t already considered it, take the time to analyze your campaigns through the lens of these response codes. They are valuable allies that will help you better understand what works and what doesn’t in your email deliveries, allowing you to improve.
