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So, suggestions (in ascending order) are:
1. Delete & reinstate contact
2. Delete & reinstate email account
3. Uninstall & reinstall Mail app

At this point them's yer best bets! 👍🏼
Yes, I guess you are correct. Thanks to everyone here for chiming in with suggestions. I have deleted and re-made the contact details. I’ll wait a while and see what happens.
 
Spoke too soon. One of these rogue emails has appeared. Again in my draft folder.
 
Good question. I have not deleted this latest rogue email and yes it does exist on the gmail website. I have had a look around the gmail settings on their webmail and I can’t see anything that might be causing this. But I am no expert on this subject.
 
I don't have an explanation of how macOS Mail or the Gmail server would create a draft email addressed to the same person spontaneously and over and over…but knowing where it originates will narrow that down.
 
Start making a log of what apps are running when the issue occurs. Any app can request that an email window pops up with the address, subject or body prefilled with default data, so there's a possibility that an app on your system other than Mail is triggering this.
 
I don't have an explanation of how macOS Mail or the Gmail server would create a draft email addressed to the same person spontaneously and over and over…but knowing where it originates will narrow that down.
Are you (the OP) using Notification alerts?

For some time if I clicked the alert to dismiss it instead of using the x in the top left corner Mail would open a reply instead of the message it was alerting me about.

That behaviour seems to have stopped and that action now does as expected, and opens the message window.
 
Are you (the OP) using Notification alerts?

For some time if I clicked the alert to dismiss it instead of using the x in the top left corner Mail would open a reply instead of the message it was alerting me about.

That behaviour seems to have stopped and that action now does as expected, and opens the message window.
I am using notification alerts. But the contact we are talking about, is not very tech savvy and rarely, if ever, will they send me an email unless responding to one I sent.
 
This week I got my own rogue draft email message! It's in an IMAP account that's hosted by a simple web hosting service (Cpanel). I have two Macs, one iPhone and the webmail portal as ways to access the account. It keeps reappearing after deletion.

What I've tried today (and we'll see if it works) was having Mail open on both Macs and also the iPhone and the webmail portal open in a browser. I deleted it from every one of those at the same time (as quickly as possible, that is) and I'm waiting to see if it reappears again.
 
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