One way Private Messages?

[Friz]

One of the members sent me a Private Message in response a question I posted on the Diesel Discussion. It was helpful advice. When I replied to his Private Message, saying "thank you" I got an Error message stating the recipient has chosen not to receive Private Messages. What gives? I did not initiate the correspondence. How is it you may send a private message yet the recipient is denied the courtesy of a reply? Doesn't sound right. Thanks.