A search revealed this:
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macOS Mail: Previous Recipients file location #
Modern versions: ~/Library/Containers/com.apple.corerecents.recentsd/Data/Library/Recents/
Recents
Recents-shm
Recents-wal
Legacy versions: ~/Library/Application Support/AddressBook/
MailRecents-v4.abcdmr
MailRecents-v4.abcdmr-shm
MailRecents-v4.abcdmr-wal
Importing the Previous Recipients to a new Mac is as easy as pasting the first set above and rebooting.
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Source: tinyapps.org/blog
So, to be the "test dummy", I tried this:
1. I went to the folder above "~/Library/Containers/com.apple.corerecents.recentsd/Data/Library/Recents/"
2. I located the three files in question, and moved them to the trash
3. I quit Mail.app, then emptied the trash
4. I restarted Mail, and...ALL listings in the "Previous Recipients" window are now... GONE.
Yes, it's a "brute force" job, but this will completely clear out old addresses and let you start over...