Hi,
Emails sent via Special:EmailUser will now show up with a From address of wiki@wikimedia.org, instead of the user who sent the email. This is to work around DMARC restrictions[1] that affected some users depending on their email provider (I think Yahoo was most affected).
The Reply-to address will still be set to the email address of the sender, so pressing "reply" in your email client will continue to work as expected. In addition, the footer message is a little more explicit that by replying, you are revealing your email address[2].
This should make the Special:EmailUser service more reliable, I hope.
[1] https://phabricator.wikimedia.org/T66795 [2] https://phabricator.wikimedia.org/T152242
-- Legoktm