Microsoft Exchange Server 2007 uses security certificates for client connections. This complicates the situation for Microsoft CRM 4.0 Email Router that needs to access the Exchange Server to route emails.
Microsoft CRM 4.0 Email Router is very buggy. One of the problem is that you can give it an HTTPS Exchange server address, it will try to use HTTP to access user email inboxes regardless. This will cause the access to the Exchange Server to fail.
One remedy is to use IIS 7.0 URL Rewrite Module to redirect all HTTP traffic to HTTPS. When you do this, you have to make HTTPS as "NOT REQUIRED" in IIS. This way the Rewrite Module will actually get a chance to redirect the traffic. The net effect is virtually the same as requiring HTTPS, since no meaningful responses are sent via HTTP.