You have to install IIS on any machine which you use to send mail via WebMail, according to something I read recently. I encountered similar problems, and reverted back to sending mail through Outlook.
Outlook is a bit of a pain as the security kicks in every time you try to send a mail, using a developed application, but there is a handy little utility and its freeware called CLICKYES to work around this. It allows you to send a mail without having to click Yes to Outlooks security messages, and is configurable, so you can talk to it in .NET to put it in Suspend or Ready Mode.
Sweep
...if it works dont mess with it