IIS 7 and Above
URL Rewrite Module
Enable SSL using remote proxy
Last post May 07, 2012 07:58 AM by Dalong Zhang - MSFT
May 03, 2012 02:14 PM|DionysosDale|LINK
May 07, 2012 07:58 AM|Dalong Zhang - MSFT|LINK
I guess that this is because there is no outbound rule?
Not outbound rule, it's inbound rule. You will need to create a rule forwarding requests to content server via SSL.
For more information, you can refer to the following article(focus on the second situation)
SSL off-loading in Application Request Routing