Milter-greylist

'As of October 21, users will no longer be able to post new content to the site, and on December 14 Yahoo will permanently delete all previously posted content. "You'll have until that date to save anything you've uploaded," an announcement post reads. Yahoo Groups, launched in 2001, is a cross between a platform for mailing lists and internet forums. Groups can be interacted with on the Yahoo Groups site itself, or via email. In the 18 years that it existed, numerous niche communities made a home on the platform. Now, with the site's planned obsolescence, users are looking for ways to save their Groups history.’


Time to move to Google Groups?
Or maybe groups.io… (free!) they appear to have a yahoo groups migration path which will copy all the emails, but not any attachments. It does require the yahoo group admin to set a few things. 

Description at: https://www.youtube.com/watch?v=sdgJR9ZzcgE

Bill