I appreciate that similar questions have been asked about how to setup SMTP relaying with IIS's virtual SMTP server. However I'm still completely stumped on this problem.

Here's the setup: IIS 6.0 SMTP server running on Win2k3 box with a NAT'ed IP. Company uses Gmail for all email services. An app on the box needs to send email, so normally we'd just set the app up to talk to smtp.gmail.com directly, but this app doesn't support TLS. Easy, we just setup a local SMTP relay right? So I thought.

What we have done so far: Setup IIS SMTP server to relay to smtp.gmail.com, as per these excellent instructions: http://fmuntean.wordpress.com/2008/10/26/how-to-configure-iis-smtp-server-to-forward-emails-using-a-gmail-account/

The local SMTP relay allows anonymous access.

Both the local IP and the loopback IP have been explicitly allowed in the Connection... and Relay... dialogs.

Tried sending email from 2 different apps via the local SMTP server, but failed (the emails end up in the Queue folder, but never get sent). The IIS logs show the conversation with the local app, but zero conversation happening with smtp.gmail.com. The port used by gmail is open outbound, and indeed the apps we have that support TLS can send email directly via smtp.gmail.com, so there is no problem with the network.

At this point I changed the smtp settings in IIS SMTP server to use a different external SMTP server and hey-presto, the local apps can send email via local IIS SMTP relay.

So smtp.gmail.com fails to work with our IIS SMTP relay, but another 3rd party SMTP service works fine. We need to use smtp.gmail.com, so how to troubleshoot this one?