It is required that an MTA *sending* a message (i.e. an SMTP client) query
for MX records, and then, failing that, query for A records. It has NEVER
been required that every host which is an SMTP server (or client) have an
MX record.

It has been a *recommended* practice for each SMTP server to have an MX
record, because common DNS implementations did not support negative
caching, and having the MX record was good for the Internet (less traffic)
and the DNS (fewer queries). BIND has supported negative caching for a
while now, and the reasons for SMTP servers to have MX records (aside from
the functions that MX records directly provide) are not as pressing as they
once were.

However, on the SMTP server side (the receiver of a message), RFC1123 is
*quite* clear about attemps to "verify" the identity of the sender: they
are permitted, but "the receiver MUST NOT refuse to accept a message" (page
51, RFC1123, "Host Requirements").

	Erik E. Fair	<fair@clock.org>