Oh!
And I thought this was your problem?

You cannot send at all?

Could you please try to run fmms from console (as user)
python /opt/fmms/fmms_gui.py
create a new (simple text) MMS and send me the out put in terminal. As well as the file /tmp/fmms.log?
And also the content of the newest folder (ls -el /home/user/.fmms/sent) from /home/user/.fmms/sent.

It could be your provider switched to MMS version > 1.0?
And refuses messages with version number == 1.0. It sounds absurd to me, as this will (and does) lock out a lot of devices. But hey, it is t-mobile! (similar happened to me here in germany for receiving, my provider uses now 1.1 and that specific header. Could be the reason why my wife's phone, a 6310, does not receive MMS anymore, too)
That should not be the case, as the versions are backward compatible. But being asked, I see this as most probable cause.
I will later attach two files, where I push up to 1.2 (not latest 1.3).
Just backup original ones under /opt/fmms/mms/message.py and mms_pdu.py, replace with attached (of course as root) and restart fmms.