deletion: auto-fill / recents in email

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Am thinking there is a plist or *.db file that can be deleted/edited - just need to know it's name

(I know that a user can delete them one at a time from within the Mail app)

 

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Well I made a backup with iMazing and then in the backup located the file 'Recents' in the Mail folder. Sure enough it has hundreds of entries.
I deleted the entries and saved the file, then restored from the backup.
No help, the 'Recents' still exist.
Perhaps the 'Restore' only copies files from the backup that do not exist on the iDevice?