You will receive an email with the new forum post every time someone writes something new.
To reply to that forum post via email,
1) Open the email. It will look similar to the email below.
2) RED Area: In the email, there will be a line. Reply above that line in order to respond to the forum post.
The forum post that you are responding to will appear below the line in the email.
PURPLE Area: There will also be a link in the email that leads you to the online forum in the Collaboration Center.
YELLOW Area: Below the link, the email will also include the group the forum is in and the topic the forum is regarding.