IIS 7.5: set up SMTP service, but cannot relay mail from external devices

I've got a Windows Server 2008 R2 webserver running IIS 7.5. I've configured the SMTP service, and am able to send test messages from the server itself. However, I'm trying to configure a scanner on another network to use the SMTP server to send scanned documents, and that's where I'm running into trouble.

The SMTP service is configured to use the IP address of the server. For the qualified domain, I'm using smtp.[mydomain].com, which resolves to the server's IP. I'm using dedrelay.secureserver.net (GoDaddy dedicated server relay) as a Smart Host.

In the SMTP settings, I've authorized anonymous access in access control and outbound security. I've also authorized the router and firewall IPs of the printer's network in both Connection Control and Relay Restrictions. I don't think permissions are the problem, though, since the log at C:\Windows\System32\LogFiles\SMTPSVC1 doesn't show anything other than the test messages I sent from the server itself, but if there's somewhere else I can check this, please advise.

It could be that I'm putting the wrong settings into the printer. All I'm doing is entering smtp.[mydomain].com — I've pinged that domain from the printer's network, and it resolves to the correct IP. The printer claims to have successfully sent the file, but I don't see anything in the server's SMTP log or the mail queue.

Any suggestions? If there's any other information you need about my SMTP configuration, just ask. Thanks in advance for your help!