Old email keeps showing up in Mail app after Apple Account update

Re: "New Apple I/d and email but emails still include both emails

I have deleted the old email from everywhere."

Seems it must be retained somewhere .... Wondering if you checked your own "Contacts" card(s)

Your online Apple ID Account: https://account.apple.com/

Apple Account in iPhone Settings

Try entering the old address in the "To" field and see if you can click (i) (for info), to delete it there.

Try updating iOS to newest vesion (back-up data first) or try restarting iPhone.

Thankyou for checking, and for your reply..

Re: "Any other ideas ... "

Seems that old address must linger someplace.

✽ You may have sent messages to yourself at some point ?

In the Mail app: Choose "Window" from the top menu bar.

Click on "Previous Recipients"

See whether you can remove your old address from there, or not.

✽ If you have any lists in use with Mail, it may be worth checking whether the old address is listed.

✽ Else, you might want to check your addresses and contacts on the email provider's website (eg yahoo, gmail, etc.)

✽ and if you use, or downloaded, an app from the provider, could the address be listed in there ?

✽ If you use (or have used) "Hide my email" wondering if the address could still be listed in there.

See: Create and manage Hide My Email addresses in Settings on iPhone - Apple Support

✽ Also, maybe check Safari? : Above article says:

You can also generate Hide My Email addresses in Safari and Mail wherever email addresses are required. See Use Hide My Email in Safari and Use Hide My Email in Mail. In supporting apps, you can also generate a Hide My Email address when an email address is required by tapping the email address field, then tapping Hide My Email above the keyboard.