Sending email with php sendmail not working for primary domain

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?= <>, ctladdr=auriga (1001/1001), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30853, relay=[] [], 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

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:
  2. At the end of the file add the following lines of code to handle email correctly:
  3. Save the file and exit.
  4. Update the sendmail setup in the command prompt:

    Follow the steps, I said yes to everything
  5. Restart sendmail:
  6. Try sending the email again. It should work now.