Yahoo has recently (April 2014) implemented a strict DMARC policy that will cause posts From: yahoo.com users to be bounced by many other services.

There are a few things that con be done about this, none of which are ideal.

1) If you have Mailman 2.1.16 or newer, set

ALLOW_FROM_IS_LIST = Yes

in mm_cfg.py and then set the list's from_is_list setting to either 'Mung From' or 'Wrap Message'. 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.

Or

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.

Or

3) Turn off Bounce processing and live with the problem.