Really useful ‘Everyone Writes’ article on the IBM Connections wiki:
Among the new features in IBM Connections 4.0 is the Mail-in feature or, as we call it in this article, ReplyTo, which allows you to make new posts to Forums by replying to an email notification. There are a number of configuration steps that must be performed to enable this feature, both on the WebSphere Server and on the mail server being used. This article describes how the ReplyTo feature works and the steps required to enable it successfully.