The source is from here: https://www.lifewire.com/request-read-receipts-os-x-mail-1172888

In a nutshell (shameless copy, as I need this to work!)

• Open Terminal.
• Type “defaults read com.apple.mail UserHeaders”.
• Press Enter.
• If that command returns “The domain/default pair of (com.apple.mail, UserHeaders) does not exist”:
  • Type
    • defaults write com.apple.mail UserHeaders ‘{“Disposition-Notification-To” = “Name <email@address>”; }’

    replacing Name with your name and email@address with your email address.

    • The complete line could read “defaults write com.apple.mail UserHeaders ‘{“Disposition-Notification-To” = “Heinz Tschabitscher <sender@example.com>”; }'”, for example.
• If the “defaults read” command above returns a line of values that starts with “{” and ends in “}”:
  • Highlight the entire line. It might read something like {Bcc = “bcc@example.com”; }, for example.
  • Press Command-C.
  • Type “defaults write com.apple.mail UserHeaders ‘” (excluding the outer quotation marks).
  • Press Command-V.
  • Type “‘”.
  • Insert ‘”Disposition-Notification-To” = “Name <email@address>”; ‘ in front of the closing “}” character, replacing Name with your name and email@address with your email address.
    The line might now read
    • defaults write com.apple.mail UserHeaders ‘{Bcc = “bcc@example.com”; “Disposition-Notification-To” = “Heinz Tschabitscher <sender@example.com>”; }’

    , for example.

• Press Enter.

Not Only Request but also Respect and Send Read Receipts

Unfortunately, Mac OS X Mail does not respect read receipts. If you get an email requesting a read receipt, nothing special happens.

Using some JavaScript and Mail rules, you can mimic some of the behavior and send simple receipts upon request.

These do not fully conform to the standard and will not be interpreted as read receipts by the sender’s email program. Of course, the plain language acknowledgment of receipt will still be useful.

Disable Automatic Read Receipt Requests Again in Mac OS X Mail

To turn requesting a read receipt for every message back off:

• Execute “defaults delete com.apple.mail UserHeaders” at Terminal command prompt to delete all custom headers or set UserHeaders back to what it was before you added “Disposition-Notification-To”.

Full Email Accountability and Control

For full knowledge and control over the fate of the emails you send, you can employ a certified email service or use third-party software such as iReceipt Mail.