[Mimedefang] a way to handle mail from domains with "bad" mx entries?

[Albert Croft]

I recently set up a sendmail/MIME-Defang/SpamAssassin/Razor installation 
to act as a spam gateway for several domains, setting in front of the 
destination mail servers (also running sendmail) for those domains. I 
have noticed on several occasions mail that was rejected for whatever 
reason (often because the recipient address did not exist on the 
destination mail server), and the MX for the address in the from or 
reply-to fields had been set to a host with an address in the 
RFC-defined address space that represents localhost or private network 
ranges. While I only just found a reference to the 
md_check_against_smtp_server() function (and will be looking at that 
soon to handle non-existent recipients), I was wondering if there might 
be a suggested way to check that the MX had not been set to to a host 
that was unreachable because of such trickery or misconfiguration. Also, 
if anyone could refer me to additional information on the 
md_check_against_smpt_server() function, that would also be appreciated.

Your time in considering this question, and any advice or suggestions 
provided, are greatly appreciated in advance.

Sincerely,
Albert C.