I'm not sure whether it's universally supported, but AT&T at the least actually does support responses. I can text any email address from iMessage and receive an email via SMTP from 1234567890@mms.att.net, and responding to that email also delivers correctly (and is even threaded).
Edit: One funny thing is that iMessage will detect gateway traffic from/to email addresses that are tied to iMessage accounts, and put those into a "blue bubble" chat instead of the normal SMS/MMS "green bubble" chat.
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I'm not sure whether it's universally supported, but AT&T at the least actually does support responses. I can text any email address from iMessage and receive an email via SMTP from 1234567890@mms.att.net, and responding to that email also delivers correctly (and is even threaded).
Edit: One funny thing is that iMessage will detect gateway traffic from/to email addresses that are tied to iMessage accounts, and put those into a "blue bubble" chat instead of the normal SMS/MMS "green bubble" chat.