It's analogous to hosting SMS enabled services. Your network operator must provide an MMSC (MMS control centre -- that's similar to an SMSC). Your applications use the MM7 protocol to interact with the MMSC in a bidirectional manner (that's similar to using UCP, SMPP, etc. in the SMS case).
Scenario 1: An MMS terminal sends an MMS to the MMS server. Your application uses MM7 to receive the MMS from the MMSC
Scenario 2: Your application uses MM7 to "push" an MMS to a terminal.
The MM7 specification is contained in document TS23.140 available from http://www.3gpp.org
Also, to learn more about SMS, try http://www.smsforum.net
This was first published in October 2002