Internet Security and Acceleration Server 2004 has made a significant breakthrough in publishing Secured web-sites in general and Outlook Web Accesses specifically. We don't need another exchange server for use as a front-end server if the only job we need it for is protecting our internal exchange server which holds our mailboxes.
Before we begin the publishing process, we need to make sure we already have Form-Based OWA, already working with SSL (See Configuring OWA 2003 with SSL).
1. In case the ISA server is not part of the domain, the first stage should be installing CA-Root certificate of the Enterprise CA on the trusted root certificates on the ISA server. If the ISA server is a part of the domain (which is not recommended for security reasons), you can skip to the next stage.