I found out we were getting rejected by some servers because our hostname was setup incorrectly. We had updated the host name to be a valid FQDN a long time ago in the Basic WHM setup, but we neglected to update it in the Network Setup. I am going to call the new hostname: new.mynewdomain.com The old hostname: old.myolddomain.com [good] new.mynewdomain.com now resolves an IP successfully. [good] when i type "hostname" on the server i get "new.mynewdomain.com". [good] When i send an email to my personal gmail account, and check the headers, old.myolddomain.com is nowhere to be found, just new.mynewdomain.com. [bad] When I send an email to an account on one of our other servers, it gets rejected that it can not verify the sender. When i look at the /var/log/exim_mainlog, it shows: Code: no IP address found for host old.myolddomain.com Why would the other server still be checking old.myolddomain.com?