I presume that it's because it isn't formatted correctly. It should look like this link for Bored.com's site.
http://www.bored.com/
It has to include the http and all the other relevant bits (with no errors)
Either because your email account isn't formatted to receive HTML, or more likely because the person who sent it to you didn't format the email address in HTML. In other words, they've simply typed EXAMPLE@EXAMPLE.CO.UK without using the computer languge that turns that address into a clickable link, or what's known as a "mailto". (I can't show you what those characters are here because this site will turn what I write into a clickable link!) This explains it better than I can and shows you what I'm wittering on about: http://hotwired.lycos.com/webmonkey/teachingtool/m ailto.html
Just to add: The email doesn't necessarily have to formatted as HTLM, you can also use RichText which will show up email/internet links plus one is able to use italics/bold/underline etc etc