wierd bulk_mailer error

I have a list with about 33,000 addresses.
I've used bulk_mailer for it for years.
We've upgraded to sendmail 8.13.1, and also had to recompile bulk_mailer, no
apparent problems.  Compile defaults taken, 100 recipients, 20 domains as I
recall.
On smaller lists everything is fine.
On this one large list, bulk_mailer will get part way through, seems about
10% or so, then a couple of thousand sendmail msgs get logged as follows:

 mailnull@localhost did not issue MAIL/EXPN/VRFY/ETRN during connection to
stdin

 From what I can research, it looks like bulk_mailer quit sending a valid
mail and just started making/quitting connections to the sendmail daemon.

I just re-ran the job and got the same result, many good outbound batches
like I'd expect then bang-o 2,400+ errors as fast as the screen can scroll.

I can't find anything wierd in the list itself like a garbage address or
something like that (although hey I admit I didn't personally examine all
33,000+).

All ideas appreciated.

Thanks!
Glenn.