Just an observation from recent personal experience.
I had a file of alternating From: and Date: lines and wanted to convert it
into an mbox.
formail -dem2 -s < input > mailbox
should have done it, right? Nope; formail -s took it all as one message,
even with -m1. When I edited in blank lines, the command worked.
My first reaction was that the -e option wasn't working as advertised and
that the blank lines were necessary after all.
Then I realized the real problem: there was no interruption in the succession
of valid header lines in the input for anything that could look like a body.
I could have put something other than blank lines between each pair of
headerfields and then -e would have done its job, but as long as every
additional line looked like a valid RFC822 headerfield, even if its name was
the same as one that had appeared earlier, formail -s assumed that it was
still the same message's head.