How do I validate an e-mail address?

You can use ASP to validate the *format* of an e-mail address, but not necessarily that it is a valid and working address. Here is a rather concise script using regular expressions (thanks again to Chris Hohmann for his useful RE input): 
 

<%      Function isEmailValid(email)          Set regEx = New RegExp          regEx.Pattern = "^\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w{2,}$"          isEmailValid = regEx.Test(trim(email))      End Function    ' now, test it:        Function testEmail(email)          response.write "<p>" & email & " (" & _              isEmailValid(email) & ")"      End Function        testEmail("bob")      testEmail("aaron@!whatever.com")      testEmail("aaron@whatever.com")  %>

 
And in JScript: 
 

<script language="JavaScript" runat="SERVER">  function isEmailValid(email)  {      return /^\w+([-+.]\w+)*@\w+([-.]\w+)*\.\w{2,}$/.test(email);  }    // now, test it:    function testEmail(email)  {      Response.Write("<p>" + email + " (" + isEmailValid(email) + ")");  }    testEmail("bob");  testEmail("aaron@!whatever.com");  testEmail("aaron@whatever.com");  </script>

 
For testing that the e-mail is actually valid (and not just in an acceptable format), you may want to generate some random string or a key, e-mail it to the user, and have them enter it into your interface. You can match the random string in your database to verify that it is the same string they were sent, therefore you know that the e-mail address is valid.