Mail processing hangs when encountering a message witha non valid TO address

I had noticied that the mail queue processing hangs when I have a message with a to address with a type  such as mailaddress@yahop.com instead of yahoo.com
The error is 
`[-][error][Swift_TransportException] Swift_TransportException: Expected response code 354 but got code "554", with message "554 5.5.1 Error: no valid recipients"`
