Sendmail User Unknown Error (Resolved)

Recently, I have faced the below issue with a new Sendmail installation on a Linux system. While sending emails to a specific domain like user@domain.com are showing the error “stat=User unknown“.

The issue was happening for a specific domain only. Sending emails to other domain were working properly.

The log file entries are looks like below:

Jan  7 08:36:04 tecadmin sendmail[22497]: 1078a4RD022497: from=www-data, size=679, class=0, nrcpts=1, msgid=, relay=www-data@localhost
Jan  7 08:36:04 tecadmin sendmail[22497]: 1078a4RD022497: to=user@domain.com, ctladdr=www-data (33/33), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30679, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown
Jan  7 08:36:04 tecadmin sendmail[22497]: 1078a4RD022497: 1078a4RE022497: DSN: User unknown

Solution:

After troubleshooting the issue, we found that this happens because your domain name matches either your server’s hostname or a setting in Sendmail’s config file. In that case, we can configure Sendmail to force send emails to your actual mail server instead of itself.

Follow the below instructions:

1. Edit /etc/mail/sendmail.mc and add the following lines at the end:

   sudo vim /etc/mail/sendmail.mc 
   

   Add the following entries:

   define(`MAIL_HUB', `domain.com.')dnl
   define(`LOCAL_RELAY', `domain.com.')dnl
   

   Make sure to change your domain name end with the trailing dot!

2. Now, run the sendmailconfig command, so that the changes take effect, and restart sendmail just to be 100% sure:

   sudo sendmailconfig 
   

   Press ‘Y’ for all confirmation prompted.

3. Finally, restart the Sendmail service to apply changes.

   sudo systemctl restart sendmail 
   

All done. Again send an email and the email should be delivered to the actual mailbox.