Well ... it doesn't seem to be a problem with formail, as far as I can
tell. When I extract the offending message to a file, and run
$ formail -eYd -1 -s <myfile
the output is in fact the single message I want. So it would appear
to me that the trouble is that the output of formail isn't going where
it should be going.
The problem is that procmail is writing a big file to a pipe, the other
end of which is "formail". Using -1, formail quits reading after
spliting one message, leaving the pipe full of unread data. Procmail,
not really knowing what's going on, keeps trying to write the large
digest into the pipe, and eventually gets an I/O error because the pipe
is full, and, on top of that, the other end has disconnected.
What does procmail think of this? This is typically an error, so
procmail "recovers" the input and throws away the output of the filter
command.
Using the "-i" flag on the procmail recipe, tells procmail to ignore I/O
errors on the pipe.
Try it and see.
Alan