Given the following section in Web.Config:
There is no way to specify whether a SSL / Secure connection should be used when defining the mail settings in the web config file.
I'm pretty sure that's why you get an error.
One way to solve that is to create your own mail sending wrapper, and then using it in your application, but you probably already got that :)
It gets more complicated when using the .net membership controls, like forgot password, create user etc. A way to solve that, is to override the SendingMail event, and then set the EnableSSL property of the mailclient to true.
I hope i was able to help :)