If I've got it right, any email server that identifies itself as
"mail.domain.com" necessarily can only fully support 1 domain as it is
also required to accept email in the form user@[123.123.123.123].
That is unless user "user1" on domain1 and "user1" on domain2 and for all domains are
necessarily the same user. i.e. All users must be
unique across all domains that the email server services for. In which case, the email server can be valid for an unlimited number of domains.
Or, in other words, if you want "user1" on "domain1" to be different
from "user1" on "domain2", then the email server can only fully support
1 domain per IP address.
Right?
I don't mean "will it send mail". I mean does it follow the RFC so that
the anti-spam nutcases don't put it on their black-lists.