Multimedia Messages are to be encoded using WAP standards before being transmitted to the MMSC. The Encoder can be developed by you or you can use the MMEncoder Class Nokia's MMS Library to encode the Multimedia Message according to WAP Standards.
You can download the Nokia's MMS Library from http://www.forum.nokia.com/smsforum/main/1,35452,1_2_7_2_2,00.html
My company - www.anam.com - sells a Microsoft COM based SDK that can generate MMS notification and message WAP binary encapsulation encodings. Mail me for details.
In addition to srini890's suggestion (good answer, btw), you can also use the "Nokia Developer's Suite for MMS" or the NMIT as a tool for MMS encoding and to test/validate your MMS messages.
Go to www.forum.nokia.com
Messaging >> Multimedia Messaging >> Development & Tools >> Message Authoring
And
www.forum.nokia.com >> Tools and SDKs >> Nokia Mobile Internet Toolkit 3.1