In xenForo, a member (who is subscribed by email) only receives an email for the first response. They receive no further email updates until they visit the thread and it's marked as read. I recently changed one of my sites from IPB to xenForo, and my members love xenForo! However, the one feature they really miss is the ability to receive all updates by email, instead of just the first thread response. I'd like to suggest adding this as an option for a future version.