In the native Mail client on iPhone and iPad, iOS automatically remembers email addresses you previously sent emails too. You can delete any remembered email address easily – follow these steps.
1. Open iOS Mail app.
2. Start typing a remembered email address. You will see it appear. Click the “i” next to the email address.
3. Scroll down to the bottom of the screen and select Remove From Recents.
If there is no “i" next to an email address, the email address is saved in your contacts list. You will need to remove the email address from your saved contacts email address field.