I had an interesting issue last week. I had a website with a contact page that uses SMTP sendmail to email a list of people whatever was entered on a form. The curious thing about this issue was it would send emails to everyone in the list except email addresses that matched the domain name of the web site. For example, it would send an email to firstname.lastname@example.org, but it wouldn't send an email to email@example.com, even though the server that was doing the sending was listed in the DNS as 'continuouslylearning.blogspot.com'.
I tried changing the email provider by pointing the Email Host to a server that I knew accepted anonymous email pushes, but it did the opposite - sending the emails successfully for users from firstname.lastname@example.org, but not sending them for blah@anyOtherAddress.com!
After looking through various logs (including /var/log/maillog), it seemed like sendmail was getting confused as the log was trying to associate the email address (email@example.com) with a user on the host and not finding the user.
sendmail failed error: localhost: firstname.lastname@example.org user not found
I tried removing an entry in the /etc/hosts file that pointed the IP address to continuouslylearning.blogspot.com, but that didn't seem to work. What I didn't realize until later (with the help of my hosting provider) is that this was the correct change to make, I just needed to restart two services after I made the change:
service network restart
service sendmail restart
Then my email worked perfectly.