Thanks for the quick response.

What happens is messagewall opens 5 connections to sendmail.  I'm not sure if it even issues a helo at that point.
Messagewall gets inbound mail and forwards it to sendmail through the already open connection.  if Messagewall is sending two messages to sendmail at the same time, it'll use a 2nd (already open) backend connection.  It's somewhat rare for messagewall to have to send 3 messages to sendmail at the same time (probably once an hour), so the 3rd, 4th, and 5th sendmail connections are there only for occasional bursts of volume.

As soon as messagewall starts and connects to sendmail, I have 5 child sendmail/spamass-milter processes.  After a few minutes or up to a few hours (depending on our volume), at least two of the least used spamass-milter processes die.
In the log, I don't see spamass-milter dying (not debugging yet), but during messagewall's next attempt to try the sendmail child that would've called the dead spamass-milter, I see this in the log, which seems pretty generic:
Milter (spamassassin): write(D) returned -1, expected 5: Broken pipe

My sendmail's "Timeout.command" option needed to be increased so that it wouldn't kick off the idle messagewall connections.  This has done what I needed up to this point (before spamassassin was installed), but sendmail has a lot of other global timeouts.  Not sure if any of them would have an effect on this problem.  I've also increased the timeouts where the milter is defined in the sendmail config.  I found a line in spamass-milter.cpp that said:
nready = poll(fds, nfds, 100);
I tried changing that to:
nready = poll(fds, nfds, 1382400);
Didn't really know what that would do, but I figured it couldn't hurt.  Lastly, I've tried starting spamass-milter with "-- -t 0".
No luck with any of these.

I know mimedefang-multiplexor had a timeout option for situations like this where there's an idle smtp proxy connection, and I was thinking that spamass-milter probably had something similar - even if it's hard-coded.
Mimedefang is way overcomplicated for what I'm doing.  Aside from this small problem, spamass-milter is perfect.

I need to do in spamass-milter what can be done with mimedefang-multiplexor's -b option: set the timeout.

I run messagewall, which connects to sendmail.  messagewall holds the connection open with sendmail, and the sendmail slaves that are seldom used fail when they call spamass-milter, because spamass-milter has timed out.
i've set the sendmail and milter options in sendmail.mc to a number which far exceeds the scheduled restarts and log rotations of the mail services, but this doesn't help.
The issue seems internal to spamass-milter.

It looks like this was dealt with in poll.c, but i don't know what to change.  I need the timeout to be 8 days.