I can see the message I posted to a discussion in the web UI, but I didn't receive any of the responses via email. Why is this?

In order to receive form posts via email, you must monitor the forum. You can do this by placing a check next to the forum you wish to monitor and choosing "monitor selected" from the Monitor drop down at the bottom of the Forum list.

For more information on this see the section "Monitoring a folder" of the User Guide in the link below.