There's a useful article (http://www.freebsddiary.org/relay.php) for relay function for sendmail, take a look here.
Ahh, this is postfix.... ok, the same applies, but you may want to see those...
Are you sure that local delivery should be disabled, and "relay=none" ? Double check those settings at your mail server config.
Also, grep -r for "916CCBA20" - this is the message ID containing the...