SMTP server errors and how to fix them

SMTP mail server functions

The SMTP server works according to a simple set of rules:

• The sender (MAIL FROM) and receiver (RCPT TO) are specified;
• The SMTP server sends the appropriate requests in both directions, waiting for a response;
• broadcasts messages to the specified addresses.

By default, the software uses port 25, but it is often blocked by ISPs because of the high intensity of spam. To organize the normal operation of the mail server, the standard ports are changed to 465 in case of SSL connection, or to 587 if there is a need for protection against virus mailings. Data transmission in this case is performed with mandatory sender authentication, which is unacceptable for intruders.

Advantages of the SMTP server

This protocol allows effective spam filtering, including the ability to manually limit the number of emails a particular address can send. The second important advantage is that subscriber data is stored on the sender's server, while SMTP only transmits the message without keeping confidential information. And the third, one of the most important advantages is the high speed of message delivery, reinforced by the ability to see the delivery status.

SMTP server response codes

SMTP server responses are encoded into a three-digit numeric cipher containing the results of request processing. Here are the most common ones:

• 2XX - means successful execution of the previous command;
• 3XX - the system gives such codes at intermediate stages, if the server is still waiting for the rest of the message;
• 4XX - temporary server error codes;
• 5XX - critical errors.

The second digit helps identify the response category:

• X0X - syntax error;
• X1X - response to an information request;
• X2X - error with the transmission channel;
• X3X and X4X - unknown nature of the error;
• X5X - mail server status.

The third figure essentially complements the second, allowing for more information about the incident.

Based on all of the above, there can be many variants of errors, so below we will consider only those that occur most frequently.



SMTP error 421

If you see this error, it indicates that the server is unavailable. There may be three possible reasons:

1. There is a blockage on port 25. The solution requires adding additional ports to the config.
2. Incorrect connection settings. Need to check and correct it.
3. An anti-virus system or a built-in firewall blocks a connection to the SMTP server. In this case, add the server IP address to the exclusion list.

Error 451

Indicates an interrupted message transfer process. May occur due to incorrectly specified domain MX records on the DNS server - for example, incorrectly specified preferences from different domains.

It can also indicate that the server limit for sending messages is exceeded. In this case, you should check the limit and change it if necessary.

Error 550

Signals an invalid or non-existent recipient address (RCPT TO). It may also indicate the presence of a spamming virus inside the network.

Error 571

Indicates that the recipient refuses to accept the message. There can be several reasons:

• Blocking the sender's IP address on the recipient's side;
• The sender's email has been added to the spam list;
• The sender's IP does not have an rDNS record, and without it no mail server will accept your emails - contact your hosting provider for a solution.

Error 452

The requested action cannot be performed due to insufficient space on the recipient's server. You simply need to increase the capacity on the destination side of the drive, or free up the occupied space by deleting unnecessary or outdated files.

SMTP error codes, causes and how to fix them

There can be many error codes, we have considered only the most common ones. As a rule, if the error is typical, it is sufficient to google the code and explore working solutions - there is enough information on the Internet. If you still have any questions, please contact the HostZealot specialists and we will help you with our own expertise and will advise you on how to deal with these or those problems.