Your request travels as a hidden note inside the email. The recipient’s program then sends the receipt automatically, asks the recipient, or ignores the request, depending on its settings.
Where to find it: in Outlook, in the Options of a new message, and for all messages in the Tracking settings; in Gmail, under More options in a new message, but only in Google Workspace accounts that allow it. Other providers and webmail services have the option in the compose window, in settings, or not at all.
HawkOpen works differently: it adds an invisible image and tracked links and records when they are requested, only for recipients who consented to tracking by the sender’s company. That needs no reply from the recipient, cannot prove reading either, and is announced by a notice at the end of each tracked email.
Where rules that require consent apply, tracking may start only after that consent has been obtained. The link in the message lets the recipient withdraw consent later.