Prevent Mail.app from sending read receipts?
There are two header tags related to receipts:


Return-Receipt-To This is asking for a verification that the mail got to your inbox.  I was surprised that mail responded by sending the original message back to me. macOS 10.10.5  I see this as a minor privacy issue in most circumstances. Could be used to tell if your work computer was powered on and mail app running. 
Disposition-Notification-To This asks for what you did with the email. Mojave gives you some options. I'd display what you have then delete it.
To control, Disposition-Notification-To,  you may use terminal comments document on this web page 
https://miln.eu/mailreceipt/, see below, or install Mail Receipt which is a free macOS application that adds delivery notification and read receipt requests to your e-mails. https://miln.eu/mailreceipt/


As far as I know, you cannot change how mail responds inbound "Return-Receipt-To". Mail will always echo back your message to the sender. You can control how mail responds to "Disposition-Notification-To" via commands or an app

Terminal commands:

I'd display what you have then delete it. To view existing UserHeaders enter the following Terminal.app command:

defaults read ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist UserHeaders


To set a Disposition-Notification-To header via UserHeaders enter the following Terminal.app command:

defaults write ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist UserHeaders '{"Disposition-Notification-To" = "email@address"; }'


To remove all UserHeaders enter the following Terminal.app command:

defaults delete ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist UserHeaders


Via the free Mail Receipt macOS app.