can't receive mail

[Steve Kleene <skdeb@syrano.acb.uc.edu>]

I am trying to replace my old Red Hat 6 mail server with an Etch machine.  At
the moment, the Etch machine can send mail but not receive it.  In all cases
I am using sendmail.

The server (Syrano) has a fixed IP address within the university system.  I
have both Old Syrano and New Syrano configured with this fixed address and
connect one or the other (but never both) to the net.  The old one has
functioned well with sendmail for many years and still does.  The new one has
a good network connection, and I can access its web pages via http at the
expected IP address.

The New Syrano can send mail fine but not receive it.  If I go to a second
machine on the network (Numbat) and send a mail to New Syrano, the following
sequence occurs:

  1. Numbat's maillog says "stat=Sent (Message accepted for delivery)"
  2. The mail doesn't show up on New Syrano (as of 99 min anyway).  I found
     no related entries under New Syrano's /var/log in mail logs, messages,
     or anywhere else.
  3. If I switch the Ethernet connection back to Old Syrano, the message
     is delivered there with some delay.  It appears to have been waiting at
     a mail relay within the university system.

"iptables -L" on New Syrano shows an ACCEPT policy on all three chains.
/etc/hosts.allow allows all services from Numbat (shown by its IP address,
not by name, which used to be important).  I didn't see anything obvious in
sendmail.cf or sendmail.mc, but I also don't know a lot about those.

I can start posting some diagnostics and config files, but maybe at this
point someone can suggest some obvious problems I may not have thought of.
Thanks.