also, why is MXrecord value “mail.domain.com” being suggested when there is no mail server on the system with this hostname? The webmin host doesnt even use that name (its one of the clients/virtual servers…these are not real servers), so why is this being suggested in Virtualmin? Shouldnt virtualmin be using the actual system hostname.fqdn when suggesting dns records?
I think Postfix accepts mail for all sub domains for a domain (with mail enabled). Suggesting mail.domain.com as a DNS record might just be something that’s become the norm - you could test it by making it mails.domain.com as a test and see if mail inbound & outbound works - I suspect it will.
Postfix uses entries in /etc/Postfix/virtual (which is subsequently hashed) to work out what domains to receive mail for.