Coincidentally, the app password that I've used for Fastmail's CalDAV service for years suddenly started causing 403's today. I wonder if that's related (but can't think of how it could be)
(I work for Fastmail.) One of our attempts at doing some mitigation of the attack caused this; we fixed it about 16:00 US Eastern this afternoon. Sorry about that!
Without getting into too much detail: we were limiting some HTTP methods, but unintentionally blocked REPORT, which DAV clients use to see what’s changed.
This is going to sound weird, but my father can't send email with attachments currently. I'm a long time fastmail user/customer and I'm also a long time IT guy so can't see what's going on.
Basically SMTP login and everything works fine, client sends the message and then it just times out.
Without getting into too much detail: we were limiting some HTTP methods, but unintentionally blocked REPORT, which DAV clients use to see what’s changed.