does not exist.
An internic whois lockup for that domain does not yeild a result:
and if you go to Godaddy.com and try to register that domain, it is available.
The SMTP server most likely has The "reject_unknown_recipient_domain" restriction in place which is causing the error. This will need to be turned off on the SMTP server to allow the script to complete, or the bad email address will need to be removed.