I have been stuck to this problem on Linode Server. I was not able to send mails to my primary domain. It was giving following error in mail logs.

Nov 18 13:20:06 indidelights sendmail[4820]: sAI7o6ST004820: to==?utf-8?B?Uml0dSBB?= <ronak@mydomain.com>, ctladdr=auriga (1001/1001), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30853, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown

Apache was trying to handle the email locally on our server. When I looked at the log response I saw it was returning the following error.

User unknown
550 5.1.1 recepient@example.com

After following these steps the email was routed to the correct mail server. My setup is Ubuntu 12.04 with PHP 5 Apache2 and Sendmail.

1. Edit the sendmail config file on the Apache server:

   sudo nano /etc/mail/sendmail.mc
   

2. At the end of the file add the following lines of code to handle email correctly:

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

3. Save the file and exit.
4. Update the sendmail setup in the command prompt:

   sudo sendmailconfig
   

   Follow the steps, I said yes to everything

5. Restart sendmail:

   sudo service sendmail restart
   

6. Try sending the email again. It should work now.