Presuming that the old email address is not in Contacts then:
If, when you are typing in an email address, an old email comes up as a choice then there will be a blue arrow next to it. Touch the blue arrow, scroll up, and select the red Remove from Recents from the set of choices.
The email will no longer come up as a choice.
If the email address is in your contacts, remove it from there. If it isn't Nd its just a "remembered" address, there should be a blue arrow next to the contact name in the window that pops up. Tap the blue arrow and go down to Remove from Recents and tap that.