We often get request from customers to control mail flow routing in a hybrid deployment, for various compliance requirements. Achieving the desired routing greatly depends on where the MX record is p...
Updated Aug 17, 2022
Version 4.0
Blog Post
8 MIN READ
Advanced Office 365 Routing: Locking Down Exchange On-Premises when MX points to Office 365
This is not an acceptable solution. They also fail to tell you about a bundle of other things that will be blocked by using the transport rule.
1) Delegates will not receive calendar appointments sent to them because of this rule (Exchange strips the Received: headers out and still considers the message external)
2) If you have a Centralized Mail Flow turned on so that all your Office 365 emails exit via your on-premesis servers, it will reject those messages for some reason believing they are NOT internal (Though it will still let you email from a 365 user to an Online user).
3) If you have Unified Message setup and you receive a call from someone that doesn't have an extension on your system (Like EVERY external user), Exchange doesn't enter an email address in the from and then will not consider the message Internal and it will be blocked.