Mail: outgoing and now incoming mail server rejects password

Hello all. Hope you can help with this one.

I was using Mail as normal yesterday. I use the same incoming mail server and SMTP server (my university's Exchange server). Then sending mail suddenly failed. The error message I get is:

The SMTP server “exch.wlv.ac.uk” rejected the password for user “(null)”

Enter your password again or cancel.

The settings are port 995 for outgoing mail, using SSL and requiring password authentication.

I've verified my settings and passwords with IT support. They say they haven't altered anything, and nor have I.

I've tried Connection Doctor: it says Mail connected with the exch. server without any problems. However, under Account Information, the server says (offline).

I've changed ports (to 25 and 587): no success.

I've deleted my password from Keychain and Mail plist and reentered it. No success.

I've verified and repaired permissions.

I'm now not getting incoming mail either: the error message is

There may be a problem with the mail server or network. Verify the settings for account “Staff POP” or try again.

The server returned the error: The connection to host exch.wlv.ac.uk on port 25 failed.

I can send and receive using web-based OWA.

Is there anything else I can do, or is it a server problem?

Thanks,

Aidan