iMessage text reply fails back to email

[dejahu]

I have a user who regularly emails from outlook to a #######@vztext.net or att.net for specific people who get it sent as a text to their iPhone. The # that displays as who the text is sent from is always a random Verizion auto-generated #. User can receive the text, but when replying to it they get "iMessage failed to send message". Replying to other messages sent in this fashion with other users works fine, it's only this specific person. On the iPhone, they had iMessage disabled. Re-enabled it, still fails to reply to this specific user's message. Only thing I can think of is it might have something to do with the email they are replying to is set as someone's iMessage email address and that contact saved in the address book needs to have the email address removed from the contact profile. We've been puzzled for days about this, any ideas?