Email is not a guaranteed service! There is nothing in the various IETB RFCs that say it is or commit it to being such. Messages can get stuck because network routes go down — you are likely to know that as most mail daemons will bounce stuck messages back after some period has elapsed (typically 28 days).
You could set read receipts on the message but there are users — and I am one — who deliberately will not honour such receipts to be returned. And the last time I checked Apple Mail does not respond to them either.
Back when I ran Linux and hosted my own mailer I had procmail recipes that expunged Delivery- and Read-Receipt lines from message headers and further squelched them by setting my pine options to No for both things.
Some people try to get around this reassosnable desire for privacy by including a 1x1 gif in the message but various mailers (and receipients) are wise to that and block the downloading of such nefarious things.