sendmail 8.9.3-15 (from RH 6.1, also -30 from 6.2) refuses to receive
incoming mails via SMTP when there is an empty MAIL FROM: during the SMTP
session producing the following output in /var/log/maillog:

Apr 12 16:53:17 sirion sendmail[21407]: QAA21407: SYSERR(root): buildaddr:
no host
Apr 12 16:53:17 sirion sendmail[21407]: QAA21407: from=<NONE>, size=0,
class=0, pri=0, nrcpts=0, proto=SMTP, relay=some_host [some_ip]

This usually happens when someone of our users tried to send mail to an
invalid email address. The remote MTA generates an error message which is
sent back to the sender of the original message but with an empty MAIL
FROM:

This error can be reproduced on several machines.

I used /sbin/mailconf to generate /etc/sendmail.cf. Using the vanilla
sendmail.cf directly from the RPM without calling mailconf, the error does
*NOT* occur, i.e. sendmail will accept SMTP connections with empty MAIL
FROM:.