Its because all traffic are router via Microsoft servers and all email are checked before beeing handed over to third parties....

Agreed. Suggest you tell your clients "we do not support Outlook for Cell or New Outlook for PCs because it acts as if it is spyware"

We block authentication for ALL countries except for 5.

MS servers try to hit us via IPs in USA and Ireland.  Our server blocked Ireland (as intended) but allows the US connections as one of our allowed countries.  I'm not sure Microsoft's ability to failover IPs that are not connectable but we've not seen issues as of yet.