There are a couple of approaches. You can have code in your ASP.Net site that when a user does something it sends an email.
The other approach is to write a windows service that is constantly looking at a database table to see what needs to be processed and then have it send the emails and your ASP.Net app will write events to that table.
To send emails there are tons of example online using the System.Net.Mail namespace and SMTPClient class.
http://msdn.microsoft.com/en-us/library/vstudio/system.net.mail[
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