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Powershell/PowerShell-Office365Admin/ExchangeWithAADConnect/CompareRemoteMailboxesToO365.ps1

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+# Find mailboxes for synced accounts, where the mailbox exists in Exchange Online but doesn't exist on-prem as a remote mailbox.
+#
+# Like the other stuff in this folder, it's a bit of a niche scenario.
+# We're looking for mailboxes which were created in on-prem AD and then licenced in the cloud without a remote mailbox being created on the on-prem Exchange server, so that we can remote mail enable them correctly.
+#
+# This is a bit complicated as it requires connected to two Exchange environments at once, so uses command prefixes for one of them.
+#
+
+# What is the FQDN of the on-prem Exchange server?
+$exchangeServerFQDN = ''
+
+# What is your remote routing address? E.g.: @domain.mail.onmicrosoft.com
+$remoteRoutingSuffix = '@domain.mail.onmicrosoft.com'
+
+# Find and load the new ExO "module"
+$exoModulePath = (Get-ChildItem -Path $env:userprofile -Filter CreateExoPSSession.ps1 -Recurse -Force -ErrorAction SilentlyContinue).DirectoryName[-1]
+. "$exoModulePath\CreateExoPSSession.ps1"
+
+# Establish a session to Exchange Online
+Connect-EXOPSSession
+
+# Create Exchange on-prem connection Uri from FQDN
+$exchangeConnectionUri = 'http://' + $exchangeServerFQDN +'/PowerShell/'
+
+# Establish a session to Exchange on-prem and add the OnPrem prefix to all commands
+$userCredential = Get-Credential
+$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri $exchangeConnectionUri -Authentication Kerberos -Credential $userCredential
+Import-PSSession $session -DisableNameChecking -Prefix OnPrem
+
+# Get list of mailboxes from Exchange online for synced users
+$allOnlineMailboxes = Get-Mailbox -ResultSize Unlimited | Where-Object {$_.IsDirSynced -eq $true -and $_.Name -notlike 'DiscoverySearchMailbox*'}
+
+# Get list of remote mailboxes from on-prem server
+$allOnPremMailboxes = Get-OnPremRemoteMailbox -ResultSize Unlimited
+
+# Compare the two lists and add those missing from the on-prem list to the $syncedMailboxMismatch hashtable
+$syncedMailboxMismatch = $allOnlineMailboxes | Where-Object {$allOnPremMailboxes.PrimarySmtpAddress -notcontains $_.PrimarySmtpAddress}
+
+# Find mailboxes where alias and username match
+$matchedUPNAliases = $syncedMailboxMismatch | Where-Object {$_.Alias -eq $_.UserPrincipalName.Split('@')[0]}
+
+# Find mailboxes where alias and username don't match
+$mismatchedUPNAliases = $syncedMailboxMismatch | Where-Object {$_.Alias -ne $_.UserPrincipalName.Split('@')[0]}
+
+# Fix the mailboxes where username and alias matched.
+foreach ($mailboxToRemoteEnable in $matchedUPNAliases) {
+    $mailboxUsername = $mailboxToRemoteEnable.UserPrincipalName.Split('@')[0]
+    $mailboxToFix = $mailboxToRemoteEnable.UserPrincipalName
+    $remoteRoutingAddress = $mailboxUsername + $remoteRoutingSuffix
+    Enable-OnPremRemoteMailbox -Identity $mailboxToFix -RemoteRoutingAddress $remoteRoutingAddress
+}
+
+# Write out the mailboxes that have been updated
+Write-Output -InputObject ('The following mailboxes have been remote mail enabled.')
+$matchedUPNAliases | Select-Object Name,Alias,UserPrincipalName,PrimarySmtpAddress
+
+# Write out the mailboxes that have been left due to a mismatch between username and alias
+Write-Output -InputObject ('The following mailboxes have not been changed because they have a mismatch between email alias and username.')
+$mismatchedUPNAliases | Select-Object Name,Alias,UserPrincipalName,PrimarySmtpAddress
+
+# End the Exchange Session
+Remove-PSSession -Session $session