Help configuring Domino SMTP Network

The situation:

We have two Domino 5.0.5 servers, each on a separate
400.  One 400 sits entirely on our LAN (no connection
to the outside world).  Let's call this one "A."  And
"B" is connected both to our LAN and the internet. 
Our networking guys have set "B" up with great care so
no internet IP addresses forward to the LAN.  Also,
the IP address referenced by our DNS MX record sits on
"B."

Until last week, "A" housed all user's mail files, and
"B" handled the sending and receiving of internet mail
(using AS400 SMTP, as we could not get Domino's native
SMTP to work when we installed 5.0.2 on them).  For
some unknown reason (or someone not owning up to
making a change), this has ceased to work.  Mail goes
out to the internet, but will not come in.  We never
see it on "B's" Domino console, or in its logs
(strangely enough, we don't see outgoing internet mail
on B's console anymore either).


We set this up some time ago, all notes are lost, and
the Notes administratorship has rotated among quite a
few people (and now resides with someone with very
little experience).  We've tried in vain to get it
working, and to switch to Domino SMTP (we don't really
care what we use as long as it works).  If we use
Domino SMTP, we can receive incoming mail from the
internet, but outgoing messages get stuck in routing
loops.

I know that what we're trying to do is not at all out
of the ordinary, but are there any good references on
how to set up something like this?  We've spent the
last 16 hours trial-and-erroring it, to no avail.  We
need help.

Thanks!
Mike E.

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