Maybe another setting that can impact your situation:
Do you have is consumer organization enabled in your hosting plan? (thats an option where every exchange user within the organization is 100% segregated rather then for example a company or group that work together etc.)

Marco - you're the man 🙂
Disabling "Consumer Organization Support" within the Hosting Plan fixed the problem ! I must have just set this accidentally when creating the Hosting Plan
Thank you 🙂 Have a great evening
Cheers, Scott