Actually, sometimes it is a website.
The Collegedale Credit Union's website (for example) used to be listed as http://ccu@collegedale.org/ ... which actually works, even though their mailto: link points to the "same" address.
(And I'm pretty sure it's not just a
Microsoftish "embrace and extend" autocorrecting URL entries, because it works in Lynx too--I just checked.)
I suspect that the @ and anything before it is simply dropped, so the users should still get a page, even if not necessarily what they were looking for... You can remain smug in the knowledge that you know better.
What bothers me more is the phrase "log on to our website", for websites that don't have logins.