[Mailman-Users] dangers of using subject_prefix with new DMARC problems

[Stephen J. Turnbull]

Matthew Taylor writes:

 > Is it safe to add back the subject_prefix to my lists?

No.

You can do it, but DKIM will break for the DMARC p=reject senders I
know of (ie, AOL and Yahoo!).  Then you will need to do something else
to mitigate the problem, such as use the dmarc_moderation_action
option in the most recent Mailman release to get yahoo.com and/or
aol.com addresses out of the From header.  (This is effective because
DMARC triggers on the From address.)

This has two disadvantages.  First, it becomes hard to reply to such
messages.  We do tweak the Reply-To header (which usually does NOT
affect DKIM, but could in theory), but it's not possible to get even a
majority of MUAs to DTRT.  Many users are going to lose on either
Reply-To-Author or Reply-To-All.  Depending on how users use the list,
this may or may not be a big problem.

Second, since it only happens to AOL and Yahoo! posters, they often
complain that they're being discriminated against.