Difference between Usermailbox vs Office365 mailbox - Exchange Hybrid

Copper Contributor

We have Exchange Hybrid setup. In exchange admin center we would like to understand the different between creating “User mailbox” vs “Office 365 mailbox”.


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The problem is when we create a new in active directory and it gets sync. The mail box is creatd with MailboxType = User and RecipientType = UserMailbox. However when we create it from EAC console as Office 365 mailbox the Mailbox Type is set to Office 365 and RecipientType to MailUser. We would like to understand what different it would make and if there is a possibility of creating Office 365 mailbox from AD Sync.




5 Replies
User mailbox means creating mailbox in your local DB, O365 mailbox means creating mailbox in O365.
If you login to O365 you will be able to see mailboxes there, which are displayed here as mail users.
Recipient type is "mail user" so that your on premises exchange knows that there is a corresponding mailbox for this object in O365.

In other words it creates a "remote mailbox" directly in Exchange Online.

like they mentioned, the mailuser object is created so that on-prem knows to put that object in the GAL for on-prem users and knows that it can accept mail for that address and forward it along to EOP to deliver it to O365. 


This is an older post, hopefully you can answer my question.

We are in a hybrid configuration. All of our mailboxes have been successfully migrated to Office 365. Now, when we need a new mailbox, we create an on-premise user type mailbox and then migrated to Office 365. After the migration the On-Premise EAC shows the mailbox type as Office 365. 


Instead of doing the migration, can we create an Office 365 mailbox right from the start in the On-Premise EAC? This also creates and AD account that we can then go in and add the necessary permissions, and no more migrations. Is this just wishful thinking?

You need to create user in local AD, there is no way of syncing a user from O365 to on-premises, it only supports syncing of very few user attributes.
Once you have created user in local AD, run Enable-RemoteMailbox SamAccountName -RemoteRoutingAddress User@domain.com
This creates a remote mailbox, now after one sync cycle of AAD Connect, assign the license to user in O365.
Now you have both, a user mailbox in O365 and a remote mailbox in you local exchange.