Now the discussion is getting awesome: lot of ideas and a problem solving attitude.
Sadly Office365 is strictly HTTP(S), so no luck with regards to prioritizing the TCP ports.
I hope some Shorewall black belt could shed some light.
Let me argument a little bit with regards to the O365 discussion: my full time job consists of managing a group of 15 sysadmins as IT Manager. I’m very experienced in Windows and most of Microsoft products but, since half of the colleagues I coordinate are Unix / Linux experts and all of our services run on heterogeneous systems, I appreciate both closed source with bold support policies and open source and community driven products.
I strive to pursue the correct balance between these two worlds avoiding fanboys prejudices and haters. My philosophy is “If you have more money than time, buy, if you have more time than money, make” and most of the open source solutions, to me, happen to fall in the latter category.
Just my two cents.