recently i had some issue while configuring email functionality so while exploring the solution and analysing the log file i found the functionality being executed on F9 command.
1. F9 send email command uses two method of Outbound communications Manager business service.
2. First it stores the Template body in a variable by using ExpandCommTemplate method ,which is hidden method and not much documentation has been given about it.(for more details please refer http://siebelfoundation.blogspot.in/2015/07/hidden-methods-of-outbound.html)
3. Secondly it uses send message method and uses the value it gets from parsing the template as Message body ,email from the recipient group and communication profile which user selects after pressing F9. (for details about send message method please refer http://siebelfoundation.blogspot.in/2015/04/how-to-test-email-notification-in.html)
So this the reason why HTML templates cannot be sent using F9 ,as the method ExpandCommTemplate converts template body in plain text not HTML.
Hope this was helpful. cheers ..