Odd ipfilter/postfix interaction

All:

Maybe someone who is more of a networking guru than I am can figure out
how to solve this problem.

I've got a FreeBSD 4.11-STABLE system that is attached to the Internot
via DSL. Generally, that all works just like one might expect it would.

Recently, I switched to ipfilter 4.1.5 from some relatively recent
release in the 3-series of ipfilter.

Shortly after switching releases of ipfilter, I noticed that e-mail
messages with attachments (sized approximately 116K) were not able to be
relayed via postfix (I'm running 2.1.5, if anyone is interested) off the
server. Messages would queue up on the server, and they could never
leave. The reason the messages were deferred was always "lost
connection with fake-smtp.server.com [1.1.1.1] while sending message body".

So, I tried changing the server to which I relay messages. That didn't
make any difference.

Then, I tried a different mail client for a couple of test messages, on
the off chance that the MUA was constructing malformed messages. That
didn't make a difference.

Then, I tried looking at MTU issues since that is what Googling the
problem seemed to indicate might help. I tried mucking around with MTU
settings in /etc/ppp/ppp.conf (since I use PPPoE to connect to my DSL
provider). No matter how (reasonably) low I set the MTU, the messages
would not go through.

Finally, with the same set of messages sitting in the queue that had
been failing, failing, and failing again, I dropped ipfilter and flushed
the postfix queue one more time.

Blink and you missed it. The messages were gone.

If anyone wants to try diagnosing this problem and is interested in
looking at a tcpdump of a failed message, let me know and I'll make it
available to you.

David Bogen