SOLVED: Office 365 - AD Synced user not showing in Exchange Hybrid on-premises

 How to fix Office 365 mailbox not showing in Exchange Hybrid on-premises using PowerShell Enable-RemoteMailbox

Problem: AD Synced user not showing in Exchange Hybrid on-premises

The User account has been created in AD and synced to Office 365 but doesn't exist on the Exchange Hybrid server

In this case, the Exchange Hybrid server is being used as an SMTP relay and we are getting this error when people try to copy the user into emails sent through the SMTP relay

Checking the mailbox properties, we can also see that the Exchange Hybrid user@domain.mail.onmicrosoft.com smtp address is missing

User exists in Office 365

User doesn't exist on the Exchange hybrid server

Solution: Run the Enable-RemoteMailbox PowerShell command

On the Exchange Hybrid Server - Open Exchange Management Shell

Run the following PowerShell to convert the AD user to and Exchange Hybrid user

Enable-RemoteMailbox user@domain.com -RemoteRoutingAddress user@domain.mail.onmicrosoft.com 

Run Azure AD Connect Sync

From the AD Connect server, run this PowerShell to sync the chnages to Office 365

Start-ADSyncSyncCycle -PolicyType Delta 

Reference:

Exchange PowerShell - Enable-RemoteMailbox
https://learn.microsoft.com/en-us/powershell/module/exchange/enable-remotemailbox

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