Mailtrack, and some other email tracking "services", sends a message back to the sender by attaching a pixel into the email which includes code to send information back to the sender when the email is opened. Some people have reported success in defeating this behaviour by deselecting a setting in Mail's preferences so that it does not load remote content in email messages.
To achieve this go to Mail's Preferences > Viewing tab, then take the tick out of the box which says "Load remote content in messages".
Let us know if this works for you or not.