This is the logged error shown in SQL when I try to send a test message from Database Mail:
The mail could not be sent to the recipients because of the mail server failure. (Sending Mail using Account 1 (2011-04-26T19:57:59). Exception Message: Cannot send mails to mail server. (Mailbox unavailable. The server response was: 5.7.1 Unable to relay).
This is the information from the receive connector I setup on the Exchange server:
Receive Connector: SQL Connector
Use these local IP addresses: "all available IPv4" Port 25
Receive mail from remote servers: *IP address of SQL server
Authentication: Everything is checked except for "externally secured"
Permission Groups: Everything is checked
Anything glaringly obvious I'm missing here?