It doesn't get sent to an account that the receiver has to access -- it goes straight to their phone. That's because the email address you sent it to IS their phone. They'll receive just as if it were an MMS, indeed, it becomes an MMS when handled by the carrier (e.g., mms.att.net). If they reply it will come to the email account you sent it from.
For those people that I want to send pics, I just make another entry in my contact for their MMS address. For example, if my daughter's number is 1234567899, I added an email address which is 1234567899@mms.att.net. Now when I want to send a pic to her phone, I just email it to that address. She gets it as an MMS, and when she replies it comes to my email address which I pick up on my iphone.