I've encountered behaviour in Outlook for the first time today that I've
never seen before.

I was deleting emails in my junk email folder and a message popped up
stating that a sender wanted a receipt that their email had been deleted.

I can see why they'd do it, i.e. as a means of identifying real live email
accounts, but why do Outlook even have the facility of delete receipts?

Seems only useful for spammers or am I missing something?