If you copied the details from message you recieved then you would still have the error, If you look at your error it says:
To: moodle <firstname.lastname@example.org>
The issue is that you are trying to email Moodle not the person who made the comment. The email you get about this reply will say it is from me but will not have my email address it will have the Moodle address as detailed in the link Bob Provided. If you did want to reply to me via email without it going on the forum you would need to make sure the to field had my individual email address.