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Office 365: Hybrid without everyone DirSync

For an acquisition strategy we may have a tenant were 100 mailboxes will be in the cloud and 1500 will be on-premises.

Normally this is no big deal, right?  

But in this case we only plan to DirSync (AD Connect) the 100 AD Users with the mailboxes in the cloud.

Inbound mail will hit on-premises and that should work fine.

My question involves what we will have to do to get internal email to travel from the cloud mailboxes to the on-premises mailboxes?  Will that require conditional based routing with a transport rule or will the hybrid created connectors be enough... or must we sync all AD Users with a mailbox.

Note: we do not require a unified GAL

Thank you.
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Vasil Michev (MVP)

8/22/2022 - Mon
Vasil Michev (MVP)

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Satish Auti

I worked in same scenario, you just need to take care of send and receive connectors.. your domain must be authoritative in O365 portal..
Guessing that you have MX record already configured.....
If you have dirsync then you can see all users in portal but your on-prem users will not have any licensed assigned.
If your domain name is same in cloud and on-prem so the GAL will remain same...
Amit Kumar

When your MX is pointed to O365 then you can create connectors to route domain specific e-mails to your internal SMTP/Edge/gateway. So in any case e-mail is delivered to O365 and mailbox is not in cloud so it will route e-mail to on-prem gateway. Connectors will be send connector using smart host.

Rules are not needed here and even won't work as expected.


Just to clarify.. this is simply setting the domain to internal relay correct?

The connector should be in place from the Hybrid Configuration Wizard right?

There is no other configuration?

Thank you.

Notes:  Internal relay – Selecting this option means that recipients for this domain can be in Office 365 or your own email servers. Email is delivered to known recipients in Office 365 or is relayed to your own email server if the recipients aren’t known to Office 365.

I was worried that most will not be DirSynced but apparently Internal Relay is designed for this?
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James Murphy
Vasil Michev (MVP)

Yes, the idea is that you have *some* users in O365, and some in another system. When the domain is configured as InternalRelay, O365 will first look for a matching recipient inside ExO, and if such is not found, redirect the message to where the MX points at. Or over a connector, etc.