If an old, unwanted email address keeps popping up when you compose a new email message, the old address is probably still in your contacts. Follow these steps to fix it.
- In your web browser, navigate to contacts.google.com
- In the search box at the top, start typing the old address or name. Select the name from the autocomplete.
- While viewing that contact record, click the three-dot button at the top and select "Delete"
That old address will no longer appear in your autocomplete.