Per issues discussed in T134886: Special:EmailUser emails sometimes not delivered due to bounce (and the many tasks linked throughout), whereby users at certain domains cannot sucessfully send any emails via Special:SendEmail due to DMARC issues explained best at T134886#2291395
and until any alternative solution can be implemented...
- For any user who has a email address that ends in [list at bottom]
- Prevent usage of Special:EmailUser for that user
- and give them a warning message on that page, that reads: [please improve this draft!]
It is not currently possible for users with email addresses that end in [list] to send emails, due to technical complexities. Please see a technical explanation at https://phabricator.wikimedia.org/T66795.
List: @yahoo.com, @yahoo.it, @mail.ru,
(other?) (wildcard all of yahoo?)