I currently have sendmail configured on a web server, and only need to send emails for newsletters, contact forms, notifications, any e-mail address using a domain hosted on the server fails to send. We are using another server on a different IP address for e-mail, and I've tried using that as a relay but ran into the same issue. Additionally, the e-mail address is getting replaced with the FQDN.
Before I tried relaying to our mail server, I could successfully send to any domain, but had the same issue with the hostname of the webserver being added the email address (email@example.com would be changed to firstname.lastname@example.org)
Here is the current error: 050 550 5.1.1 : Recipient address rejected: User unknown in local recipient table 050 554 5.5.1 Error: no valid recipients