1. Not getting a read receipt cannot be interpreted as meaning the
receiver has not read the message. They might have turned
receipts off.
2. Getting a read receipt cannot be interpreted as meaning that the
receiver did read the message (let alone understood it).
To sum up: read receipts are a misfeature, except in the special case
of internal mail where they are part of corporate policy.
In the good old days when SPAM started we were advised to disable read
receipts for external mail because it would advertise that the address was
valid. Whenever I set up a new email client I send a test message to
check that the address works and is not sending read receipts. I
SPAM I see now often has an address for bounces -- I've wondered
about generating bounce replied to SPAM (in addition to forwarding the
message to "abuse@<internet giant>.com"). Firefox lets you set
read receipt handling for 3 categories: messages with your address in the
"To" or "Cc", "Sender outside your domain", or "other cases". I have all
set to "ask" and very rarely get asked. I do get SPAM messages with
phony "From" addresses using the domain of one email service I use.