- Open the Exchange Admin Center on the on-premises Exchange server.
- Click Mail flow, and then click Email address policies.
- Select the email address policy that you want to change, and then click Edit ().
- In Email Address Policy, click Apply to, and then add the domain (<domain>.mail.onmicrosoft.com) to the policy.
- Wait for directory synchronization to run. Or, force directory synchronization. For more info about how to do this, see Synchronize your directories.
For more info about how to edit an email address policy, see Edit an email address policy.
For more info about the coexistence domain that's added by the Hybrid Configuration wizard, see the Domains entry at Hybrid configuration options.