Could you humor us and install chomp SMS and see if MMS works? If I recall correctly, doesn't Handcent use it's own SERVER in the middle to send MMS? Maybe it's them causing your issue...
Ive used chomp ever since cm10 came out for the RAZR and never had an MMS not go thru...
In regard to Handcent using it's own server, I'm not 100% positive but I think by default that is disabled. There is an option to send it through their servers if you have a res or large file size items you couldn't normally send via MMS - they refer to it as Handcent MMS+