So this is a great question. Apparently this is a KNOWN issue with the iPad and iPhone. I found some info on this in the Apple Forum.
Solution 1 https://discussions.apple.com/message/12529236#12529236
iBackupBot for iTunes. http://www.icopybot.com/itunes-backup-manager.htm
I would recommend doing a backup of your phone directly before running the program, as the program pulls your latest backup.
Once open, you will select Plugin from the top menu and then select Recent Email Address. This will pull email addresses you have sent to. You can delete them individual. Once done, close out the pop up.
You will then select the icon for Restore Backup/File to iPhone/iPod touch. This will do a restore, only making the changes you selecting. Your iPhone will automatically restart. If you test one of the emails you deleted, you will see they no longer show up.
Solution 2 https://discussions.apple.com/message/11691816#11691816
For MAC users - You should be able to do this easily on your computer (not on your iPhone).
Open MAIL on your Mac. Click on Window, then on Previous Recipients. Highlight the name(s) you wished removed; alternatively, click one name, then choose Edit/Select All. Finally, to remove those selected, click on 'Remove From List' on the small Previous Recipients dropdown window. The next time you sync your iPhone, all should be fine.
May 17 '11 at 14:50