Recently after trying to get our server off of the blacklisting, we had an issue with WordPress giving us your host may have disabled the mail() function… This turned out to not be centralized to WP, but to sendmail. It wasn’t the fact that sendmail wasn’t able to send emails out properly, because the fact was that we were able to send mails anywhere EXCEPT to our main domain; so we were able to send emails out to gmail, yeahoo, etc., but unable to send to johnathankong.ca only. In looking at the logs for the sendmail (/var/log/mail.stats) I noticed that we were getting a lot of “DSN: User unknown” errors. after stumbling through multiple sites, I realized that the change I made to the sendmail.cf file was causing the issue. Apparently when I set the Dj to point to my main domain, it disabled all emails to that domain through sendmail. My solution, workaround, was that I created a production.mydomain.com and put it in place there. Now all my emails are up and running perfectly.