I have a N900 and today, whenever I send an SMS (to ANYBODY), as soon as I click on SEND I get:

SMS Delivery Failure for XXX


this hapens pretty much as soon as I press SEND, suggesting somethings up with the SMS message centre settings on T-Mobile for the N900.
The weird thing is, that I haven't done anything with the settings, it just refuses to send SMS's to anybody. The Phone works fine, so does the Internet on 2G and 3G.

Any suggestions?