Edit "Previous Recipients"
I have an old e-mail address that I have removed from my Mac's Mail Previous Recipients, and have removed it from my Addresses application.
How do I remove it from my iPad's Mail previous recipients?
iPad, iOS 4.3.3
Unfortunately you can't remove them. They will continue to pop up and you will just have to ignore them. The mail app on the iPad does not rely on email address from your address book or contacts but rather "remembers" email addresses that you previously typed or responded to or received mail from.