"mail loops back to me (MX problem?) Local configuration error"

This is a very common problem covered in the Sendmail FAQ:
http://www.sendmail.org/faq/section4.html#4.5

If you receive these errors in Sendmail, class w--the list of hostnames for which the local computer will accept mail--may not be configured correctly.

Example error:

   ----- The following addresses had permanent fatal errors -----
<root@host.server.com>
    (reason: system config error)

   ----- Transcript of session follows -----
553 5.3.5 host.server.com. config error: mail loops back to me (MX problem?)
554 5.3.5 <root@host.server.com>... Local configuration error

Solution (Sendmail version 8.11 and greater):

1. Add host.server.com to /etc/mail/local-host-names
2. Send the Sendmail daemon a SIGHUP signal (ex. kill -HUP `head -1 /var/run/sendmail.pid`

Solution (Sendmail versions before 8.11; I would recommend upgrading to a later version of Sendmail):

1. Add host.server.com to /etc/mail/sendmail.cw
2. Send the Sendmail daemon a SIGHUP signal (ex. kill -HUP `head -1 /var/run/sendmail.pid`)

These solutions assume you are using the use_cw_file feature in Sendmail, and that you are using default locations for the files containing additional class w addresses. Check your sendmail.mc or sendmail.cf files to determine if you are using this feature and the locations of the files.

You can display class w entries with the following command:
echo '$=w' | sendmail -bt -d0.4

By default, class w contains localhost, the IP address 127.0.0.1, and the system's IP address(es), fully qualified domain names, and short hostnames. Entries placed in /etc/mail/local-host-names or /etc/mail/sendmail.cw are added to these default values.

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Last modified: 03/06/2003