Common causes: the recipient chose never to send receipts or declined this one; their program ignores receipt requests; an Exchange or Microsoft 365 administrator blocks them; the message has not been opened yet; or the receipt landed in a junk folder. A delivery receipt also depends on the receiving server, and many servers do not send one.
HawkOpen’s tracking image needs no answer from the recipient’s program, but it is added only for recipients whose consent to tracking is recorded, and it has its own limits: image blocking, Apple Mail and scanners. HawkOpen shows every image load and click with a category and rule code, and every tracked email carries a visible notice.
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.
Sources
- https://learn.microsoft.com/es-mx/answers/questions/5578209/directiva-en-exchange-online-para-confirmaci-n-de
- https://learn.microsoft.com/es-es/answers/questions/4590545/no-recibo-confirmaci-n-de-lectura-de-correo-enviad
- https://learn.microsoft.com/fr-fr/answers/questions/5902990/les-accus-r-ceptions-ne-sont-plus-notifi-s-du-tout