DMARC change to mailman lists

DMARC is a technology designed to combat forged email coming from
senders other than those who are entitled to send as a particular
domain.

Unfortunately there are times where you may want to legitimately “forge”
the sender address of an email. eg on mailing lists. Typically if a
poster sends a message to a list it arrives from their actual email
address, eg jbloggs@some.domain.org.

The mailing list software then sends that email to all the members of
the list, and depending on the list settings, usually as the original
sender’s email address, in this case jbloggs@some.domain.org. So if
this list is hosted at inf.ed.ac.uk, then our mailserver has to
“forge” the email to look like it has come from the @some.domain.org
domain.

Through DMARC the owners of some.domain.org say inf.ed.ac.uk is not
authorised to send mail as @some.domain.org, and anti-spam filters
will take this into account when deciding to deliver the “forged”
email.

This situation is now affecting the use of some of our lists, eg if
they contain non-inf.ed.ac.uk addresses, and those members post to the
list. The lists involved could be changed so that all posts to the list
appear to come From: listname@inf.ed.ac.uk (rather than the original
sender), but then all replies would automatically go to the list,
which is not usually what you want.

The mailing list software we use, mailman, has an option to detect if
a poster is posting from an address using DMARC, and for their posts
it changes the From: field to be listname@inf.ed.ac.uk, and sets the
Reply-To: their original address. This should then keep the anti-spam
filters happy, and still mean that replies would tend to go direct to
the poster, rather than the list.

This setting is now the default for new informatics mailing lists,
and shortly we will be retrospectively enabling this setting for
existing lists.

If you are a list owner, then you check the setting under Privacy
options -> Sender Filters -> dmarc_moderation_action

Neil
Services Unit