It is not just you. We are experiencing the exact same problem, starting at the same time (Friday afternoon). Since then, a lot of our emails are being rejected with "Relay access denied" errors (or similar relay messages).
It is becoming a real pain, especially because it is intermittent and we're losing important communication among staff.
We've submitted several follow-ups on the original trouble ticket that we first generated on Friday, so I think tech support is still working on it.
I'm glad to see a confirmation here by someone that it is not our problem alone either, which means it is more systemic and deserves escalated attention - thanks for posting your message.
In one reply from tech support we were advised by to use port 587 on SMTP (instead of port 25), but this has not helped. Perhaps it will do more for you. ???
Is anyone else out there experiencing this?
We're hoping for a quick/permanent resolution,