I cannot firewall the PC as i will have 2 wan ports in use on the modem and one will have strict firewalls and the other will have open firewalls, So that when the vessels come into port and can connect via 3g internet connection they can do what ever they need via the internet.
This is exactly why you SHOULD firewall the PC, it's a simple software switch.
You could even get someone who can do some basic coding to make a nice "SAVE DATA!" button in a simple app, and you maintain the block lists for them.
You could use Peerblock and host your own blocklist?!
EDIT: in addition, Peerblock can ALLOW HTTP, and block all other protocols.
So the end user, wouldn't see any loss in "Web Browsing" ability, you'd just block the background apps.
I block "Microsoft" in my blocklist, but allow HTTP and never hit problems.
We have tried to use software in the past but the users have managed to fuck it no matter what, Also if someone gets on the wifi on the boat a software block on the computer will not prevent them from accessing the internet pages. The system also needs to be easily changed over if a new pc is put onboard if its a hardware based firewall it is plug and play but if its software it could be a little more in depth. A few companies have been using software switching but it has not been working out for them and they are also looking into similar setups to us.
I suspect Office 365 won't work properly or will echo annoying messages if it can't do it's back-to-base bollocks.
I also suspect that it's probably not much extra overhead anyway. Imagine millions of connected computers bombarding the servers every couple of minutes, I don't think they'd want that.
I tend to agree - try an alernative email client that doesn't do any licence verification.
By overhead do you mean data usage? If so i have done some testing with data usage and have found that over the course of a month office 365 used around 30 Mb of data for its back to base pings, Which on the satellite data costs around $2100 ($700 per 10Mb).