Thank you, this actually makes some sense. It's disturbing to get these emails with zero explanation what's going on, and very frustrating that user community has to guess. But PHP vuln in a web client intercepting user logins is the only thing that makes sense, otherwise that would mean passwords in the clear or some other 1990s-era nonsense.
As a precaution they reset everyone’s email passwords, presumably from an assumption people use the same ones for both.
I’m surprise they still let people use FTP.