Haven't tried the fix as I have been able to send mms but this is in the tweaks sticky.
FIXES
MMS Send Fix

Code:
Go to messages>all messages>menu>settings>options>menu>MMS server>click on SampleMMS> Server name: T-Mobile MMSIP: 216.155.165.050Port:8080Server Address: 216.155.174.84/servlets/mms or http://mms.msg.eng.t-mobile.com/mms/wapencConnect via: Tmobile MMS
MMS Size Fix -

CAB (Thanks to DaDoctor) - http://rapidshare.com/files/36980559...MSFix.cab.html

or

Enter this in your registry:
Code:
HKLM/Software/Jataayu/MMS/SP/UAProfilechanged to: http://www.htcmms.com.tw/claro/diamo...erAgentchanged to: NokiaN95/11.0.026; Series60/3.1 Profile/MIDP-2.0 Configuration/CLDC-1.1
Or

import this registry using a registry editor:
http://rapidshare.com/files/36975907...msfix.zip.html