My understanding of this system: Each of us can choose to turn on this feature in our profile. If someone sends you a private message, you can respond to them in kind only if they also have this feature turned on, or you happen to have their e-mail address. Otherwise, you have no way of privately corresponding with that person as they did with you.
I want to reply to a private message I received but I don't know how to do that. The private message said for me to go to the sender's profile, presumably meaning the page I get when I click on a member's name which provides information about the member composed by him. But I don't see anywhere there giving me access to private messaging. Thanks, grandpaw