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 …

The RFC claims underscores are removed to promote interoperability. I suspect that clause harms interoperability more than it helps.

The logic is that a large proportion of mail servers will reject messages as specified in that paragraph, so if you want to avoid mysterious delivery failures, your mail software and its configuration should follow suit - better to discover the problem sooner rather than later.