Are we talking about outlook.com or Outlook, the application
When you say "new client" do you mean a new user on the (existing) network, or do you mean a new Meraki Network you deployed at a new customer?
What does your network look like, I'd like to know what the path looks like from the client towards the internet. Something like: Client -> Access Point -> Switch -> MX -> ISP router -> Internet. Or is there another firewall in the chain?