The level of enlightenment by a colleague to find a pattern in this has saved me from a new depth of madness. My hope is that the brain trust here can now tell me why...
The Scene: We have several generic mail IDs with procmail setup to redistribute mail based on the original recipient. For example, mail sent to generic_ID1 goes to staff1,staff2; mail to generic_ID2 goes to staff3,staff4; et cetera...
GENERIC_ID1_RECIPIENTS="staff1@domain,staff2@domain" :0 * ^TO.*generic_ID1 ! $GENERIC_ID1_RECIPIENTS
The Problem: Mail was being truncated, seemingly at random. The pattern finally discovered showed that it occurred when a sentence ended with a period at column 76 and a hard carriage return: not at column 75 or 77; not at column 76 with more text to follow on that same line. Any text following that period was lost.
Addendum: we've just seen it reoccur (and reproduced it) with a period at column 226. I am gobsmacked.
I am able to confirm that procmail receives the message whole by copying the message before redelivery:
:0c # note: 'c' * ^TO.*generic_ID1 ! $GENERIC_ID1_RECIPIENTS
I believe that sendmail may be truncating it upon redelivery but I'm unsure how to diagnose or prove that (I am not a sendmail admin, just a procmail user).
The problem is entirely reproducible.
The Questions: Why is this happening and how can I fix it?
EDIT: Updated title and tags. The solution is in the comments.