Hi @Guy Edwards ,
By research, this is usually caused by the user in question having administrator rights at some point, which changes the security inheritance on their account. You can try the following steps:
- Open Active Directory Users and Computers.
- Click on View and Select Advanced Features
- Select a mailbox (User Account) that isn’t working with Active Sync, double click on the account.
- Click the Security Tab and then the Advanced button.
- Highlight Exchange Servers and check the Include inheritable permissions from this object's parent, enable inheritance.
- Click OK to save the settings.
If the above do not work, please check your active sync access rules and provide more details.
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