On March 24 I used "Email this user" on enWS to email another user, asking for a copy. Today I've received 9 (and counting) copies of that email, and the original recipient reports getting the same. It started around 08:15 UTC today, and last came in 10 minutes ago.
That sounds to me like a hickup either down in the email transport (MTA), or in the jobqueue / cron / whatever that connects MediaWiki with the email transport. I haven't seen any other users reporting the same problem on-wiki, so I don't know if it's just this message or a general problem. But it could point at an awful lot of email getting resent to an awful lot of users.
I unfortunately am not able to create a private paste (it seems you need special permissions to do that) with the full email headers. But the only obviously relevant datum in there before the email hits pobox.com's MXes (who then forward it on to my GMail account) is that the sending server is "wiki-mail-eqiad.wikimedia.org" coming on IP "208.80.154.91" and is reporting its sender to be "wiki-enwikisource-[redacted]@wikimedia.org". SPF etc. all report ok, and the Received headers do not indicate this is going through any unexpected systems.
The Date header reports today's date and the time the message was actually sent (i.e. 17:02 UTC for the last message).