As much as I feel the pain and understand the need for such functionality, I have to warn you against using it. The nature of the email is such that you can’t recall an email. What’s sent is sent. There are rare exceptions when it works as advertised, but in most cases it doesn’t. And the reason for that is – you don’t know what email client the other party uses.
Wonder, what happens when you recall a message from someone who doesn’t use MS Outlook 2003? I’ll tell you. The person gets another email, saying that you want to recall that last message you sent. And you look silly, if not annoying.
Even if the other party uses MS Outlook 2003, you still have a chance to be annoying. The thing is that you can recall only those emails that haven’t been read. But many people look through their inbox before reading every email in detail. So, imagine, they look throught their inbox, see a message from you with an important subject (you do use subjects, don’t you?) and plan their actions accordingly. First, they go through some other messages and then come back to yours. But yours isn’t there anymore. How does the person feel? Confused and annoyed at the very least. Where’s that message? What’s going on? Oh, c’mon!
Instead of recalling emails and hoping that it works, I suggest another way. Assume that recalling doesn’t work and behave accordingly. This assumption will help you spend a bit more time writing and checking your message. And if you do make a mistake (we all do), just send a follow-up message with a short apology and the part you forgot to include. That will work much better, I promise you.