While hybrid exchange environments are awesome for stretching your on premise exchange topology to Office 365, they do introduce a bunch of complexity – primarily around user creation, licensing, and mail flow.
I recently had an issue at a client where they had email bounce-backs from an on premise service destined for a few Exchange Online mailboxes. For some reason, these few mailboxes didn’t appear in the on-premise exchange environment (as remote Office 365 mailboxes), so exchange was unable to route the emails destined for those particular mailboxes.
In general, you should be creating your mailboxes on premise (Enable-RemoteMailbox), then synchronising via AADConnect – that way the on premise environment knows about the mailbox and it can be managed properly. This client was actually doing this, but obviously the process broke somewhere along the way for a few mailboxes.
There’s a bunch of different options on Google about how to get the mailbox to show up on premise – with a lot of them recommending to remove the mailbox and start again (er… how about no!).
I came across this Microsoft article on a very similar issue, but for Shared Mailboxes created purely in Exchange Online. Looking at the process, it looked like a modified version may work for user mailboxes – and it does. Below is a quick and dirty powershell script that can be used to fix a single mailbox:
#Specify who we're working with
$UPN = "firstname.lastname@example.org"
#Local exchange server
$ExServer = "Server1.local"
#365 Domain - for remote routing address
$RoutingDomain = "mydomain.mail.onmicrosoft.com"
#Connect to 365 Exchange - only import select cmdlets so they don't conflict with the Exchange On Premise session
$RemoteSession = New-PSSession -ConfigurationName Microsoft.Exchange `
-ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $(Get-Credential) `
-Authentication Basic -AllowRedirection
Import-PSSession $RemoteSession -CommandName Get-Mailbox
#Connect to local exchange - only import select cmdlets so they don't conflict with the Exchange Online session
$LocalSession = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri "http://$ExServer/PowerShell/" `
-Authentication Kerberos -Credential $(Get-Credential)
Import-PSSession $LocalSession -CommandName Enable-RemoteMailbox, Set-RemoteMailbox
#Get the Alias and ExchangeGuid from 365
$Mailbox = Get-Mailbox $UPN
$Alias = $Mailbox.Alias
$ExchangeGUID = $Mailbox.ExchangeGuid
#Create a remote mailbox
Enable-RemoteMailbox $UPN -Alias $Alias -RemoteRoutingAddress "$Alias@$RoutingDomain"
#Set the Remote Mailbox GUID to match the 365 mailbox GUID
Set-RemoteMailbox $Alias -ExchangeGuid $ExchangeGUID
Get-PSSession | Remove-PSSession