Mailbutler is a plug-in for Apple's macOS Mail client that adds some very helpful features that I use regularly. Most notably, I use it to add a delay to the sending of emails (currently ten seconds) and to be able to determine whether an email has been opened by its recipient(s).
Having a short delay when sending can be very helpful as I occasionally think of something I meant to add after clicking "send". In these cases, I'm able to cancel the send, make the required changes, and then initiate a new send.
Being able to track recipient opens can be very helpful for making sure that important emails have been received and not lost in a spam/other filter or buried in the recipient's high email volume.
There are some other features that I make use of, albeit less frequently. For example, if I mention the word "attachment" and don't attach anything to the email, Mailbutler will ask me if I intended to include an attachment.