For Windows and Outlook fans, it is possible to activate the acknowledgments with Mail on Mac OS X. I tell you first : this is a hack. Mail does not support acknowledgments by default and responses may not reach. And unlike Outlook (randomly), it is not really possible to configure the acknowledgments : in other words, either all messages have the demand, or none.
Finally, and personally, it bothers me to have to answer directly to the people’s mail.
The command is (after leaving Mail):
defaults write com.apple.mail UserHeaders '{"Disposition-Notification-To"="your@address";}'
To remove the acknowledgments (be careful if you have other UserHeaders):
defaults delete com.apple.mail UserHeaders
