What email program are you using? Have you tried another one? My guess is that it may not be working in one, but works in many others.
You might also want to clarify what you mean by “not working”. Not showing up? Not styled correctly? Clicking it shows an error? Clicking it does nothing?
Ok well anyone who has been doing HTML emails for any period of time will tell you that Outlook is always the odd one out on things working correctly. So try another email client, try gmail or another desktop app and see if the problem is there. If it is not, then you know it is something about Outlook that has a problem and you have narrowed the issue down considerably.
Secondly, tell us a bit about how you are building the email. Is this code you are pasting right into the email or are you using some kind of program or tool to build emails dynamically?