For some reason, using the localhost option in web.config to use the internal SMTP server didn't work properly, so a few people that registered never got their confirmation emails.  Two problems:

1.  Is there logging information available anywhere that would be useful in solving mail related problems?

2.  I temporarily fixed the problem by pointing the software to another server - is there a way to force a resend of the confirmation email from the admin section?  If not, there should be a way to do that.  Mail can fail for a number of reasons, including issues on the client side.

In general, this problem has to do with SMTP configuration. 

Please open your SMTP Virtual Server properties by the IIS control panel and right-clicking on SMTP Virtual Server.  Click on the “Access” tab, then click on “Access” and “Relay” and verify that you have permission to both access and relay through the server.

Please click on the link below for an in-depth Article on IIS SMTP Virtual Server Configuration that can help you with this.

http://www.dotnetjunkies.com/Article/7D8C8892-397A-400B-AD22-188B8F4F53C9.dcik