Comment by techcode

This. And it actually doesn't even need to be done through SMTP.

MMS being basically SMS with a link/url to where the phone fetches multimedia part from - it could also be sent via older EMI-UCP (that was originally used for pagers).

At some point pre 2010 (when I worked in Routo Messaging - now called Telesign) we also got an SS7 connection - so we could finally start doing stuff like a real mobile operator/provider.