OK, disclosure - I sent mms from N900 to my wife's Nokia 6133 and we both on TMo. I didn't try to send to ATT because it had a problem of communication between TMo and ATT in MMS, at least in past. I will try to research it. However, to send MMS from one TMo N900 to another TMo (non N900) in the same city I used this config: APN: T-Mobile Internet MMSC: http://216.155.174.84/servlets/mms Width: 300 phone: (my cell-phone number starting area code w/out dashes) "T-Mobile Internet" - the name of GPRS/3G connection from "Settings/Internet connections", not IP address or DNS name. It hasn't any proxy actually. And I have only one GPRS connection - didn't try fAPN to install another one.