Comment 4 for bug 1929786

I had this issue as well. Seems to be some sort of config issue. I replaced with /etc/postfix/main.cf with a blank file, and it worked.

I did not configure it myself, but I did get a question about postfix during my upgrade, where I selected the default selection. The upgrade was from 20.10->21.04, so I think this might be an upgrade issue.