[Mimedefang] Requiring FQDN in HELO

[Dirk the Daring]

   I've noticed a lot of SPAM does not have an FQDN in the HELO. It'll
have just "localhost" or even omit a hostname entirely. Obviously, if
the HELO is an IP address in square brackets, that's fine.

   Is there any danger of rejecting "legitimate" E-Mail if I write my
mimedefang-filter to:

	1) Absent an IP address in square brackets, require a
		an alphanumeric string in the HELO

	2) Reject a HELO where the alphanumeric string is not
		a FQDN (using a regex looking for at least
		one "." in the HELO string)

   The ultimate goal to allow filter_sender to reject any E-Mail where
the HELO is a blank string (spaces) or something like "localhost"
without any qualification. Are there any reasons that legit,
reasonbly-standards-compliant senders would do those things?