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This section describes how you can resolve some sendmail–related error messages. You can also refer to http://www.sendmail.org/faq.
The following error messages contain two or more of the following types of information.
Cause: What might have happened to cause the message
Description: What the user was doing when the error message occurred
Solution: What you can do to fix the problem or to continue with your work
451 timeout waiting for input during source
Cause: When sendmail reads from any source that might time out, such as an SMTP connection, the program sets a timer to the value of various Timeout options before reading begins. If the read is not completed before the timer expires, this message appears and reading stops. Usually, this situation occurs during RCPT. The mail message is then queued for later delivery.
Solution: If you see this message often, increase the value of various Timeout options in the /etc/mail/sendmail.cf file. If the timer is already set to a large number, look for hardware problems, such as poor network cabling or connections.
550 hostname... Host unknown
Cause: This sendmail message indicates that the destination host machine, which is specified by the portion of the address after the at sign (@), was not found during domain name system (DNS) lookup.
Solution: Use the nslookup command to verify that the destination host exists in that domain or other domains, perhaps with a slightly different spelling. Otherwise, contact the intended recipient and ask for a proper address.
550 username... User unknown
Cause: This sendmail message indicates that the intended recipient, who is specified by the portion of the address before the at sign (@), could not be located on the destination host machine.
Solution: Check the email address and try again, perhaps with a slightly different spelling. If this remedy does not work, contact the intended recipient and ask for a proper address.
554 hostname... Local configuration error
Cause: This sendmail message usually indicates that the local host is trying to send mail to itself.
Solution: Check the value of the $j macro in the /etc/mail/sendmail.cf file to ensure that this value is a fully qualified domain name.
Description: When the sending system provides its host name to the receiving system in the SMTP HELO command, the receiving system compares its name to the sender's name. If these names are the same, the receiving system issues this error message and closes the connection. The name that is provided in the HELO command is the value of the $j macro.
For additional information, refer to http://www.sendmail.org/faq/section4#4.5.
config error: mail loops back to myself.
Cause: This error message occurs if you set up an MX record and make host bar the mail exchanger for domain foo. However, you fail to configure host bar to know that it is the mail exchanger for domain foo.
Also, another possibility is that both the sending system and the receiving system are identifying as the same domain.
Solution: For instructions, refer to http://www.sendmail.org/faq/section4#4.5.
host name configuration error
Description: This is an old sendmail message, which replaced I refuse to talk to myself and is now replaced by the Local configuration error message.
Solution: Follow the instructions that were provided for resolving this error message, 554 hostname... Local configuration error.
Cause: When you try to send mail to a user, the error Username... user unknown is displayed. The user is on the same system.
Solution: Check for a typographical error in the entered email address. Otherwise, the user could be aliased to a nonexistent email address in /etc/mail/aliases or in the user's .mailrc file. Also, check for uppercase characters in the user name. Preferably, email addresses should not be case sensitive.
For additional information, refer to http://www.sendmail.org/faq/section4#4.17.