Hmm, very odd.
I'm not sure how it works on Bell, but on AT&T you can send an email to your_10_digit_mobile_number
.att.net or your_10_digit_mobile_number
For example, if your mobile number is 123-555-1234 then you email email@example.com
and you'd get it as an SMS (text only).
Or if you send a message with a picture attached to firstname.lastname@example.org
you'd get it as an MMS (text+picture).
If incoming MMS message works, then it's definitely an app setting about sending.
Also make sure after you've activated
the appropriate Message Center. It should have a check-mark next to it.
Same with the Access Point, Internet Profile and Browser Account. That should cover everything.
Top Menu > Settings > Applications > Messaging > Multimedia Msg > Message Center
Top Menu > Settings > More > Connection > Access Points
Top Menu > Settings > More > Connection > Internet Profiles
Top Menu > Settings > Applications > Browser > Accounts