I setuped an email for domain, like firstname.lastname@example.org, and it works.
I mean it received emails fine.
But also i setuped email forwarding to some gmail account from email@example.com
And seems like server wants to forward that email with wrong user.
2019-12-20 22:54:04 Received from <> R=1iiQDj-0004Qy-O8 U=Debian-exim P=local S=1958
2019-12-20 22:54:04 root@debian-101-buster-64-minimal R=unknown_user_router: Unknown user
*** Frozen (delivery error message)