So you’ve just realized that every time you try to send someone an email, the mail app present you with an old address instead of their new one. It autocompletes the mail recipient with an email that may not even work anymore even though you know the new email. Or it may just be that you have spelled an email incorrectly in Apple Mail App and although it bounced back, it also keeps appearing in the mail apps autocomplete function. Whatever the reason, you need to delete an email that’s old or incorrect to stop it from autofilling.
from Andrew Brett Watson.com