How does one disable the automatic sending of read receipts in Mail (MacOS High Sierra)?

There is nothing to disable. Mail does not respond to read receipts or any other disposition notification whatsoever.

I wonder if it's tied into Messages...

https://www.payetteforward.com/turn-off-read-receipts-on-mac-three-easy-steps/

Is it MS/Outlook/Exchange?

https://discussions.apple.com/thread/8311542

By default, macOS Mail doesn't support read receipts — notifications that the recipient of your email has opened it; however, if you're comfortable working in Terminal, you can make the necessary modifications to start requesting read receipts from your email recipients.

To Disable Automatic Read Receipt Requests

Turning off this setting is just as easy. As above, open Terminal again. Simply type the following, then hit Enter.

defaults delete com.apple.mail UserHeaders

https://www.lifewire.com/request-read-receipts-os-x-mail-1172888