--On Sunday, 18 November, 2007 17:28 -0800
If a reverse-path is null, it SHOULD NOT be modified as
that is likely to cause a mail loop.
I think citing loop prevention as the reason for this
recommendation is a good
idea, but think this overstates things a bit - it creates a
loops are possible, but not necessarily likely. So change it
to say "SHOULD NOT
be modified as this can cause mail loops" and I'm happy.
I just dropped 2821bis-06 into the posting queue. It says
"SHOULD NOT be modified" but doesn't give a reason.
I had two reasons for that: if you (or others) disagree, it can
obviously still be changed.
(1) More often than not, we don't give reasons for rules of this
sort. I usually favor having the explanations, but have been
regularly beaten up for it.
(2) I fear someone coming along and saying "well, I know how to
avoid mail loops, so that statement doesn't apply to me". There
are reasons for not messing with null return paths, including
their being an important clue for various automated programs,
that has nothing to do with mail loops (at least directly). So
it seems to me that, in this case, the right answer is "just say
'no'" rather than trying to justify that decision in the text.