If that address keeps popping up, it could be that the address is in the Previous Recipients list. A list of addresses that Mail and Messages (and even Phone) apps use for auto-completing addresses, and to bypass blocked mail filtering. Mail automatically adds addresses you send email to (deliberately or accidentally) onto that list. Addresses which can stay on that Previous Recipients list until you explicitly remove them.
I'd recommend checking the list, and -- if it is on that list, removing it. The problem with doing this is: you can't view or edit that list on an iPhone or iPad. To view (and edit) the Previous Recipients list, you must use Mail on a Mac.
In Mail -> Windows [not Settings] choose Previous Recipients to see the list. Select entries, and use the Remove From List button to remove a recipient's address.
[I have a Reminder, to -- as a New Year's resolution -- review that list once a year; my list has accumulated entries dating all the way back to 2009.]