I had to do some serious work to get this.. I believe its in 1.8 however, this was my method.
1. "Create admin option to allow staff to enter email address of those allowed to send HTML"
2. "Modify fetching parser to save html copy of inbound email if it exists, and if sender is allowed, otherwise, as normal, save text copy"
3. "Modify outbound parser to detect tags and allow them for certain users"
This is my version of class.mailfetch.php's getBody function: http://pastebin.com/xcq7J8LG
(for some reason new forum doesn't let you paste formatted code, so pastebin it is)
You can see I have a custom "class.format.php" function added too, stripBadHTML.. ;-)
Format: = http://pastebin.com/3c5qwcap
I modified the outbound parser to be more lenient, while at the same time, being more exacting for plain-text messages.. as I liked pre-formatted plain-text, it keeps tabs and whatnot.. useful for orders when you get a tab delimited table and actually want to know what the numbers MEAN.. ;-)
Format: = http://pastebin.com/HfEX61ZX
For you, I would start with an array of allowed senders, personally, I wouldn't be allowing everyone!