Some of the settings in the Message Formatting block are mutually exclusive. For example, you can check the box for "Force HTML Emails" and also check the box for "Plain Text Only," and save your settings without any error message. In such a case it is which setting takes precedence is unclear.
I suggest something like the following (using three radio buttons and a checkbox)...
( * ) Force HTML Emails
( ) Plain Text Only
( ) Outgoing Email format unchanged from incoming
[ ./ ] Normalize HTML Emails (this checkbox entry greyed out or absent if Plain Text Only is selected)
Others may have a better solution, but I hope this gets the point across.
Thanks for your consideration,