This has been bugging me for awhile. Occasionally (like twice this week for example), someone will tell me they really wanted to comment on one of these messages, but when they tried to reply to the email they received, the reply bounced, and no comment was sent.
Well, I've figured out why, and precisely how this website system I'm using works.
If you want to reply ONLY TO ME, and not have the reply show up on the website, you can now just reply to the email that you receive, and I will get your reply in my private email account (now that I've given the website my proper address...)
If you want to reply and have your comment sent to the group (either the entire group of former students, if the post is in the Site News section, or just the folks in your class if the post is in the Forum for your class), then click on the link in the email that says "To reply click on this link". In that case you will need to log into the site before posting.
You can tell if the post is from your own course when the email subject line starts with "AS1xx", which is the course number you are registered in.
If you've forgotten how to log in, your user name is just your last name, and the password, unless you've changed it, is astronomy.