Yahoo and AOL has recently (April 2014) implemented a strict DMARC policy that will cause posts from Yahoo.com and AOL users to be bounced by many other email services. These bounces can cause other members of your list to be removed while the offending Yahoo.com/AOL member is left alone. Because of this, we recommend that our list administrators gently encourage their Yahoo.com/AOL users to sign up to their mailing lists using a different email provider.
With that said, we have the following suggestions for our list administrators that will help them to work around this major headache that Yahoo.com and AOL.com is causing:
1. Use the from_is_list option. We have made this option available to all of our mailman users. The option is accessible on the General Options page near the top. Note that the ‘Mung From’ setting arguably creates non-RFC 822/2822/5322 compliant messages in that the From: address is now the list and not the post’s author, and the ‘Wrap Message’ setting will create messages that some mail clients, notably Apple iOS iThing clients, but others too, will not display well.
2. Turn off Content filtering, and Reply-To: header munging and remove subject_prefix, msg_header and msg_footer so Mailman doesn’t make message modifications that break Yahoo’s DKIM signature.
3. Set the list’s anonymous_list setting to Yes. This will work for the same reason that Mung From works, but it’s more drastic as it completely hides the poster’s identity unless he/she identifies himself/herself in the message body.
4. Disallow posts From: yahoo.com and aol.com addresses and tell those users they need some other (freemail provider or other) address to post. You can do this for specific known domains with header_filter_rules or, in 2.1.18, for all domains with specific DMARC policies with dmarc_moderation_action and dmarc_moderation_notice settings. (Note: mailman 2.1.18 is an upcoming release and has not been deployed as of now on our servers.)
5. Turn off Bounce processing and live with the problem.
We understand that this policy change from two major email providers have caused headaches for thousands of mailman list administrators. We also realized a large portion of these mailman administrators are using servers running an older version of mailman. We recommend if you cannot upgrade your mailman version to at least 2.1.16, to move your lists to our mailman hosting service. We are running version 2.1.17 as of April/2014 and we are committed to keeping our mailman servers up to date.