I had an interesting request from a customer the other day where they were synchronizing Active Directory into two disparate environments–Office 365 and another hosted Exchange environment. In their new Office 365 environment, they didn’t want any address proxies matching a particular pattern to be part of a user’s proxyAddress array–BUT–they also didn’t want to remove them from their on-premises accounts since they are being used by their other hosting environment as an application routing address.
Fortunately, AADConnect can do this for you. There are two was to do it (both through the GUI and via PowerShell). I’ve provided a PowerShell script that you can run at the end, but we’ll go through the Synchronization Rules Editor way first.
Read more at source: https://blogs.technet.microsoft.com/undocumentedfeatures/2016/06/21/remove-an-unwanted-proxyaddress-pattern-from-users-via-aadconnect/