http://www.abuse.net/relay.html

This is a mail relay test facility provided by the Network Abuse
Clearinghouse. It can be used in either anonymous user mode, in
which case it will send commands to the mail server, but will not
attempt to send an email message, or in registered user mode, in
which it will attempt to relay a message to the tester's address.

--
Paul Russell
Senior Systems Administrator
University of Notre Dame