I had this stupid error the other day on Directadmin. I upgraded Exim to the latest version and then all mail sent to firstname.lastname@example.org which was on this server was bounced with the error “Unrouteable address”. From the past, I knew that errors like this can be due to exceeded mail limit. But in this case, this was happening only for one mail out of 50 others, and no limits were reached.
After some searching, I found out that there was an alias pointing to the email address itself, which was causing this error. I removed the forwarder (alias) and incoming mail started to work again on this mailbox.
2021-05-09 20:53:28 H=([22.214.171.124]) [126.96.36.199] F=<email@example.com> rejected RCPT <firstname.lastname@example.org>: Unrouteable address
So I had mail forwarder like:
email@example.com -> firstname.lastname@example.org
So when dealing with such an error, just remove the alias like described above.