Don't Break Email!
From Social Patterns
If you’re using email as a broadcast medium, to send reminders or notifications for example, and not enabling people to reply to the messages they receive, that’s lame. There’s no reason why you can’t handle replies properly, forwarding them as further notifications to the correct recipients, thus balancing your interest in hosting and facilitating communication among members and between your service and your customers with your users’ interest in being able to respond to email messages using their existing habit and customs.
For example, at one time email messages sent from 37 Signals’ Basecamp product were one-way only, labeled with a warning message to the tune of “do not reply.” They got wise, though, and as long as you reply above a certain line they take your reply and add it to the comment thread on Basecamp, which serves their interests (keeping the conversation on the site) and yours (being able to hit reply like a normal human being).