Note that mail domains are subtly different from hostnames: - fully qualified hostnames can have a trailing dot; mail domains must never have a trailing dot -

Note that mail domains are subtly different from hostnames: - fully qualified hostnames can have a trailing dot; mail domains must never have a trailing dot - hostnames can be unqualified (dotless), but RFC 2821 accidentally forbade dotless mail domains so they are even more of an interop minefield than you might expect - NETBIOS hostname conventions (- forbidden, _ allowed) tend to leak into internet hostnames more than mail domains Both are subsets of domain names. I have found it useful to be pedantic about the differences, but I have worked as a postmaster and hostmaster …