Snaffled from Antony Doe

Settings / Connections / GPRS / BT GPRS MMS
Connects to:WAP Network
Access Point:Btmobile.bt.com
User Name:bt
Password: bt
Authentification Type:None
Primary DNS:0.0.0.0
Secondary DNS:0.0.0.0
IP address:(this field is blank)

By the way, if you are having problems sending or receiving MMS messages but everything else works OK check which networks are being used. e.g. BTMobile(HTC) to Orange, HTC to HTC, O2 to HTC etc. If you can send to yourself but not to/from other people then follow the link below

See also http://www.beta.bt.com/bta/forums/message.jspa?messageID=30608#30608 which refers to reporting the issue