The mailing list for listmasters using Sympa

["Mail administrator, Otto Makela" <address@concealed>]

I've come against occasional randomly failing DKIM signatures when
M365 (outlook.com etc) users send messages to Sympa mailing lists
hosted with us. Unfortunately, if the sender domain has a DMARC
rule of p=reject, this will of course mean the email gets rejected
or quietly dropped by the list recipients.

I know this has to be an issue with Sympa and/or Postfix, since
according to our incoming email spam filtering, the messages do
arrive to us with a correct DKIM.

I suspect this has something to do with the message formatting
(fairly complex multipart/related messages), but I've yet to nail
the exact details down. Unfortunately, the few samples I've obtained
from message senders' Sent folders do not yet include a DKIM signature
and seem to also have other minor changes made inside M365 before they
get DKIM signed and handed to our incoming email server.

Has anyone else seen this kind of issues?

PS. Yes, I could always give up and just have list remove the sender
and DKIM sign using the list domain, but that isn't the question here.

--
address@concealed (Mail Administrator, Otto J. Makela)