domain main email issues

Consider this actual situation:

User1 of my domain sends an email to Bob who is hosted elsewhere...

User1 for some reason attaches a blank file with extension .url, but not virus related. I think he was trying to send an Internet link. But anyway...

Bob's host rejects emails with .url attachments because of virus concerns, so the email is returned to TCH.

Where does the returned email go? I would think it should go back to user1 so he would know his email did not go through. But instead it goes to the master domain email (the email account with the cpanel username).

So, user1 thinks the email went through. Bob never got the email. A potential disaster is upon us...

But, I know the email did not go through. What am I supposed to do? Forward the email to user1? He may be uncomfortable knowing that I read his outgoing message (not intentionally - it was returned to the master domain account).

And, I don't want to have to be responsible for sending returned mail to the appropriate user.

Why doesn't it go directly to the user that sent it? Some returned mail DOES go to the returned user, other returned mail goes to the domain email account.

What is the actual use of the domain email account? What type of returned email goes to it? What type of returned email goes back to the user? What am I supposed to do?