Are your and the recipient's clients using accounts on the same Exchange server (or server farm - I'm a bit hazy about the details). The add-in sets a few flags in messages headers which, I understand, are passed on only via Exchange and only to clients
connected directly. (The final component in this system is the recipient's client - the flag settings are honoured only on desktop Outlook, and don't need the add-in to be installed.)
As examples: if I use my Microsoft corporate email account and send to another person within Microsoft, then the no reply settings propagate as expected; if I send to a user outside the corporate Exchange network, the flags settings are lost in the transition
from Exchange to whatever system the other person uses; and, finally, if I send from a different email account (eg, Hotmail) then, no matter who I send to, the flag settings don't make it any further than my machine.
Could any of that result in the behaviour you're seeing? Or have I broken something again? :-(
It would be nice to have some mechanism that doesn't depend on this Outlook-connected-to-same-server system but the starting point for the project required that recipients
didn't have to install the add-in and to do that, I have to rely only on what Outlook and Exchange offers. Maybe I can make the add-in so compelling that everyone installs it, and then I could devise some custom headers which propagate through any
email server to be processed by the add-in at receive time - but that's something for another day...
