Reverse DNS is not a valid hostname [closed]

Solution 1:

Basically, will the reverse DNS pointing to a subdomain of the domain the mail server reports be valid?

No. Just give your server a full name like myserver.mydomain.com. Make sure your reverse DNS also contains myserver.mydomain.com, and that the mailserver announced itself (with HELO) as myserver.domain.com as well.

Technically, you could have it be mail.domain.com, but that means the non-FQDN hostname of the machine would be mail, which is not elegant.

Don't have your hostname be domain.com, (I feel that) the domain should be the organizational entity, not a hostname.

Solution 2:

Both the hostname that the mail server software reports and the reverse DNS entry are expected to be the actual canonical fqdn hostname (as discussed in the referenced question for the reverse dns case).

However, it's usually not verified that these two values actually match (even though it makes more sense if they do).

Do note that there is no expectation for the hostname specified in either of these places to necessarily have any relation to the domain names that the mail server accepts mail for or sends mail from; it identifies the mail server itself, not the domains it handles.