It seems like this has nothing to do with misconfiguration and that something is seriously broken because of the need for escape characters in the etc/postfix/virtual file:
See this thread:
“Postfix ‘reject_sender_login_mismatch’ rule not usable because of escaped @ characters in /etc/postfix/virtual”
I can “fix” the issue if I remove the escape character and play around with the main.cf a little, but then I either get a relay denial if my domain is absent from “mydestination” in main.cf, or a user unknown bounce if it’s present.
In my opinion, Postfix is quite frankly lousy and Virtualmin should adopt something else. I don’t have this issue at all with whatever DirectAdmin or cPanel use for aliases (or a catch-all account, in this case)…
This is not a complaint, but merely an observation.