Quoting Michael Paoli (Michael.Paoli@cal.berkeley.edu):
So ... in reviewing /var/log/exim4/rejectlog* files, I discovered mailman-loop alias to be missing 8-O ... anyway, added that 2017-09-09. I guess I somehow missed that earlier step (probably *somewhere* in the mailman setup documentation ... apparently also for Debian, Debian doesn't have an automagic way of adding email aliases - probably mostly due to the variety of MTAs Debian supports).
I'm pretty sure you're mistaken. It's handled by an automatically created entry in /var/lib/mailman/data/aliases , a file that Mailman manages for you.
At least for now it's aliased to postmaster (which gets delivered locally) - probably at least "good enough" for now.
I _think_ you should let Mailman do it as per above.
Here's a post by Mailman chief coder Mark Sapiro re: mailman-loop address: https://mail.python.org/pipermail/mailman-announce/2008-July/000117.html
Looks like mailman-loop is mostly used for mailman administrivia emailings - e.g. password reminder mailings - and not used for, e.g. typical/general list postings.
It's for _bounces_ of password reminder mailings. The special sending address is used just in case those messages also bounce, to prevent formation of a bounce loop. You really shouldn't fool with it.
(e.g. somewhere on the Announce list, there's apparently a subscribed email address that generates an annoying backscatter ... and there isn't enough data in the received backscatter to identify the subscribed email address that's generating the backscatter).
VERP to the rescue. Enable it, at least long enough to identify the problem child. (Long term, might leave it off, as there are disadvantages. See first link about that.)
https://wiki.list.org/DOC/So%20what%20is%20this%20VERP%20stuff http://www.gnu.org/software/mailman/mailman-install/node27.html