In my present context, mail works only when the reverse is provided instead of mail.domain.tld, for some reason.
But again, scratch my present context, what I’m after is the bigger picture, that would help every time.
it’s why I’m after this kind of information, not what it is supposed to be, by the books, but what it IS - according to the virtualmin internals of each precise server causing problems.
If it is, or if it were, gathered in a determined location that the admin could read, it is, or it would be, the solution to all those email problems.