Are you setup with a data plan? With cellone and a data plan bigger than 1MB you get infospace which will allow you to receive email on the phone as well.
What is the difference in purpose between receiving an email via text ? are you trying to get an attachment?
The mailbox on the phone would be where you can setup pop3 access to your yahoo or hotmail account. That's how mine is configured.
the cellone mms address is only used by the routing system they have setup. never got the @mms one to work either.
updated 12/20/04*******
got the attachments to work to the phone now. looks like they've changed their routing. Use "1nnnnnnnnnn@mms.dobson.net" (nnn=areacode and number no dashes) and it will work. Also found that now when you email from a computer with an attached photo, it will go through as long as total mssg is less than 100k