This problem has just appeared today, at one small branch office, consisting of 2 users.
the 2 users have their mailboxes on different servers, but they now are both unable to log on. the user/pass prompt comes up after opening outlook, but it will not accept the password, it just keeps prompting (the password is definitely correct - I have tested it in OWA).
Interestingly enough though, it only works in OWA when accessing the site via the external IP of the site (port forwarding allows access). When you try and access OWA via the inernal server name or IP address, it doesnt accept the password.
Any ideas? I'm puzzled!