Yahoo recently changed their DMARC configuration, which has two results that are relevant for us:
1) It's no longer possible to send e-mails From: email@example.com, even if it's allowed from an SPF point of view (i.e. 'firstname.lastname@example.org via wikipedia.org'). This means Yahoo users cannot send e-mail from the wiki anymore.
2) It's no longer possible to change parts of an e-mail (e.g. the subject to add a mailing list name) sent by a Yahoo user. This means Yahoo users cannot send e-mail to a mailing list anymore. If they do try to, they will receive a flood of error mails from mail servers rejecting the e-mail.
See http://www.ietf.org/mail-archive/web/ietf/current/msg87153.html for more info on issue 2).
For issue 1), we might want to block Special:SendEmail for people with a @yahoo.com address, telling them their e-mail will not be delivered.