I would like to second this question.
Whenever I open iMessage, it prompts me to fill in the MMS email address in the settings, but there is no indication of what email address would be appropriate. Do I need to put in the carrier provided email address, which I have never used even once in over three years, or can I put in any of the email addresses I commonly use?
Also, how is this email address exactly used by iMessage? I don't want my private email address showing to clients, and I don't want my business email address showing to family and friends.
In the past, the cell phone number was sufficient for MMS. Why has this changed?