I recommend Privnote as a free alternative to paid by-subscription email tracking services, which I've found to be worthless or worse (worse because of misplaced reliance the user may place on them). Instead of tracking your email, Privnote tracks when the recipient reads a note to which she's referred by a special link you include in the message.
Privnote promotes itself as a means to send notes that "self-destruct," and that's true enough. In my view, though, its value lies, as mentioned, in its tracking capability. A secondary benefit is the air of importance assumed by a private note that can be seen only via the link.
The self-destruct feature can be marginally useful if you want to be sure nobody else has read your note. And before the note's been read you can use that feature to cancel the note, should you have second thoughts.
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