Notify if email is not delivered

Feature Request Summary: 

I looked around the forums to check if anyone else have this problem, but i couldn't find any.

If I put in a public reply on a  ticket, and that enduser does not have a email registered on hes/her user, Zendesk dosen't report back that it can't deliver the email, nor that end user dosen't have an email associated. This can be challangeing for us, since we have to check the user every time we send a public reply.

There seem to be no trigger conditions that can notify the agent about this.

Business impact of limitation or missing feature:

This is critical for our business as our agents.