Along with blocking those outgoing messages once you receive a message or open it, you can ask your email to request a receipt when you send a message. Again, it's one of those preferences you set.

I tried the "send receipt" function a long time ago, and remember that I was quickly swamped with little messages saying such-and-such message had been received, and sometimes another saying that it had been opened. I remember there was a standard message from AOL that said something along the lines of "your message has been opened but this receipt gives no indication of whether the recipient read or understood the contents."

SRS