Many email clients let you request a return receipt. The recipient can respond to that request, and you can be sure they got the message (even before they actually reply). In some cases, this is useful.
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In almost all cases, requesting a return receipt is but an annoyance, however. It is annoying to have your email client ask you about return receipts whenever you look at a (non-critical) email, and it is a privacy problem to have your email client send return receipts automatically.
That's why you should use return receipts only sparingly, when they are necessary.
