If you use your N900 with a new SIM that wasn't in another phone befor (or wasn't used there to send/receive MMS), make sure before you try fMMS that it would work on a phone that's capable of sending MMS out of the box.

The reason is that some operators seem to put restrictions on MMS to avoid messages being lost if the phone on the receiving end isn't capable of MMS or isn't correctly configured.

Happened to me: I played with the setting and got really frustrated until T-Mobile told me my SIM*wasn't yet activated for MMS. They activated in while we talked, and I used fMMS from that moment on successfully without a single change in the configuration.

Just as an input for those who're struggling...