Prevent Mail.app from sending read receipts? I just recently realised that Mail.app sends out "read receipts" without my knowledge if someone sends an email with a "Disposition-Notification-To" header to my work (MS Exchange) email account. This definitely seems to happen on Mail's side and not on the Exchange server's side where I made sure to turn that functionality off (also in the 'Mobile Devices' settings). 

There doesn't seem to be any GUI to turn this behaviour off in Mail.app, but maybe someone out there knows of a hidden preference that can be set? Note that if I read my email using the iOS Mail app, then no read receipt is being sent.
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.