I hate this error "The email has been used."
Yes I can understand you don't want the wrong person getting the emails.
I would rather a "Warning" "This email has been used, are you sure you would like to proceed?", rather than an "Error" "you can't do this!!" .
A small company who may only have one email and would like to look LARGE has multiple extensions / mailboxes can't have all emails going to one address.