Microsoft Outlook has the option of requesting a read receipt for emails you send.  If your recipient reads the email you sent, a message gets sent back to you, assuming the recipient allows the read receipt to be sent. 

People at my job send out stupid emails.  And they send a lot of them.  And they send them to about 2000 people at once.  Every time I receive a mass emailed message that asks if I want to send a read receipt, I consider clicking no because the sender likely doesn't even know me, let alone care if I read the message or not.  Instead, I click yes, hoping the stupid sender is overwhelmed by 2000 read receipts all at once.  Have I mentioned I hate mass emails? #technology