Every time you go to type an email address into the iOS Mail app, you get a giant list of everyone you've ever contacted that might fit the letters you've typed. This can get annoying very quickly. Here's how to delete those from your phone for good.
If you're using iOS 6, this is an easy tip for getting rid of those addresses. When you see one that you don't want saved in your recents, just tap the blue arrow next to it. From that new screen, you can just tap the big red "Remove From Recents" button to get rid of it completely. It's obvious once you think about it, yet I never thought to actually click on that blue arrow to see what was behind it.
Sadly, there's no way to mass delete recent addresses, you have to do it one by one -- but it's better than nothing. Hit the link to read more.