We deploy hybrid exchange using the HCW, AADC has been deployed as well. Mailbox A is created on exchange online. Mailbox B is created on exchange on-prem.

Now I can send an email from A to B. But I cannot send an email from B to A. It says "We won't be able to deliver this message to A because the email address is no longer valid"

A friend told me, it is because A is created on the Exchange online, so B cannot find it. Or the other hand, B is created on on-prem and have is synchronized to the cloud by AADC, so A knows it and can send email to it.

Is it true? In my opinion, once a hybrid exchange environment has been deployed, the exchange online and exchange on-prem should be able to talk with each other. So there should not be an issue when you send email across online and on-prem.

If my friend is true, what should i do the synchronized the mailbox from online to on-prem?

If my friend is wrong, which part of the setting could be wrong?



When you are in a Hybrid deployment, then you have to create the AD Account locally, ideally using EAC using the option to create an Office365 account. Then let the account sync to the cloud and licence it. If the entire account and mailbox was created in the cloud (so there is no local AD account) then it will not work correctly.

  • Then for those accounts have to be created in cloud, for example, those group accounts created by system when you create teams or sharepoint sites, is any way you can synchronize them back to exchange on-prem? – thomasnli Nov 17 '18 at 2:56

You can refer to the following article to merge an Office 365 account with an on-premises AD account: [How to merge an Office 365 account with an on-premises AD account after hybrid configuration?][1]

[1]: https://www.codetwo.com/admins-blog/how-to-merge-an-office-365-account-with-an-on-premises-ad-account-after-hybrid-configuration/#no-local-account --A user has an Office 365 account and no local AD account

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