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Office 365 Exchange Online / On-Premise hybrid configuration: Mailbox user missing on on-premise exchange server.

Last Modified: 2019-01-31
Office 365 Exchange Online / On-Premise hybrid configuration: Mailbox user missing on on-premise exchange server.

I have a couple of Office 365 accounts that were setup within active directory that do not appear in our exchange 2013 server. My guess is the accounts were created in Active directory Users and Computers, then the e-mail addresses were manually assigned to the user objects attribute "proxyAddresses". Then the accounts were synced using the DirSync / Azure Connect sync utility and the accounts were then registered with Office 365. However our on-premise exchange 2013 server does not list the Mailbox users. This is starting to cause problems with users that were setup this way. For example, we have a couple of devices that send messages directly to the on-premise server, which is routed to the mailbox within Office 365. The on-premise exchange server is unable to route the message because the server doesn't know the mailbox user exists. I found a couple of articles that direct me to make a few changes within the attribute editor. But either the settings do not work or the user account within Office 365 is moved to the deleted users. Any suggestions on how I can enable my on-premise exchange server to recognize the mailbox users?
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Distinguished Expert 2019

when you have exchange 2013 server, what is point in manually adding Proxyaddress attribute to AD users ?

Its defeating purpose of onpremise exchange

For new users You should enable / create "O365 mailbox" with onpremise exchange, then only it can be listed as mailbox with onpremsie exchange, it is nothing but MEU with mailbox hosted in O365
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