I am subscribed to a list run by LISTSERV. The other day I changed my subscription settings to have LISTSERV not mangle the message headers. After I did that, some folks complained that they were receiving return receipts from me.
That’s odd, I thought, I wasn’t asked to send a return receipt. So I dug around in the Microsoft Outlook preferences and found these tracking options.
One trick that spammers employ to verify whether or not a recipient address is valid is to request a return receipt. Most mail user agents do not return receipts without asking the user, but apparently Ouchlook has been quietly sending receipts for some time.