I've worked and consulted in over 15 very large enterprises. While it may not constitute a survey of the entire Fortune 500+, I've come into contact with enough business & IT professionals to know that is is - in fact - not a well-received feature. It's used by either underperforming folks as a CYA measure or other folks who have side-swiped by self-serving individuals.

There is no legitimate use or business-case for a read-receipt service, especially ones that use the same techniques as malware writers.

I fully support your third-party "who responded to me?" reporting & reminder tool, as I can see that being incredibly useful - especially to those who are not adept at scripting and/or using mailer APIs.