Fix Exchange Migration Error – Target Mailbox SMTP Proxy Missing – Exchange Online 2026

This method adds the missing SMTP proxy address to individual mailboxes using Exchange PowerShell commands.

1. Open Exchange Management Shell as administrator on your Exchange server
2. Connect to your on-premises Exchange environment:

   Add-PSSnapin Microsoft.Exchange.Management.PowerShell.SnapIn

3. Identify mailboxes missing the proxy address (replace 'yourdomain' with your actual tenant name):

   Get-Mailbox -ResultSize Unlimited -Filter {EmailAddresses -notlike "*@yourdomain.mail.onmicrosoft.com"} | Select-Object UserPrincipalName,Alias

4. Add the missing SMTP proxy to a single mailbox:

   Set-Mailbox -Identity "username@domain.com" -EmailAddresses @{add="username@yourdomain.mail.onmicrosoft.com"}

5. Verify the proxy was added:

   Get-Mailbox -Identity "username@domain.com" | Select-Object -ExpandProperty EmailAddresses

This method efficiently adds the missing SMTP proxy addresses to all affected mailboxes in a single operation.

1. Launch Exchange Management Shell with administrative privileges
2. Run the bulk update command (replace 'yourdomain' with your tenant name):

   Get-Mailbox -ResultSize Unlimited -Filter {EmailAddresses -notlike "*@yourdomain.mail.onmicrosoft.com"} | ForEach-Object { Set-Mailbox -Identity $_.Identity -EmailAddresses @{add="$($_.Alias)@yourdomain.mail.onmicrosoft.com"} }

3. Monitor the progress and check for any errors in the output
4. Verify all mailboxes now have the proxy address:

   Get-Mailbox -ResultSize Unlimited | Where-Object {$_.EmailAddresses -like "*@yourdomain.mail.onmicrosoft.com"} | Measure-Object

5. Force directory synchronization to update Azure AD:

   Start-ADSyncSyncCycle -PolicyType Delta

This method uses a comprehensive PowerShell script to handle complex scenarios and provide detailed logging.

1. Create a new PowerShell script file named Add-SMTPProxy.ps1
2. Copy the following script content:

   # Exchange SMTP Proxy Addition Script
   param(
       [Parameter(Mandatory=$true)]
       [string]$TenantDomain
   )
   
   $ErrorActionPreference = "Continue"
   $LogFile = "C:\Temp\SMTPProxy-$(Get-Date -Format 'yyyyMMdd-HHmmss').log"
   
   function Write-Log {
       param([string]$Message)
       $Timestamp = Get-Date -Format "yyyy-MM-dd HH:mm:ss"
       "$Timestamp - $Message" | Tee-Object -FilePath $LogFile -Append
   }
   
   Write-Log "Starting SMTP proxy addition for domain: $TenantDomain"
   
   $MailboxesWithoutProxy = Get-Mailbox -ResultSize Unlimited -Filter {EmailAddresses -notlike "*@$TenantDomain.mail.onmicrosoft.com"}
   
   Write-Log "Found $($MailboxesWithoutProxy.Count) mailboxes without proxy"
   
   foreach ($Mailbox in $MailboxesWithoutProxy) {
       try {
           $ProxyAddress = "$($Mailbox.Alias)@$TenantDomain.mail.onmicrosoft.com"
           Set-Mailbox -Identity $Mailbox.Identity -EmailAddresses @{add=$ProxyAddress}
           Write-Log "SUCCESS: Added proxy $ProxyAddress to $($Mailbox.UserPrincipalName)"
       }
       catch {
           Write-Log "ERROR: Failed to add proxy to $($Mailbox.UserPrincipalName) - $($_.Exception.Message)"
       }
   }
   
   Write-Log "Script completed. Check log file: $LogFile"

3. Save the script and run it from Exchange Management Shell:

   C:\Scripts\Add-SMTPProxy.ps1 -TenantDomain "yourdomain"

4. Review the log file for any errors or issues
5. Verify the results using the verification steps below

This method addresses synchronization problems that prevent proxy addresses from syncing to Exchange Online.

1. Open Azure AD Connect on your sync server
2. Navigate to Configure → Customize synchronization options
3. Verify that Exchange hybrid deployment is enabled
4. Check the Optional features section and ensure Exchange Mail Public Folders is selected if applicable
5. Review the Filtering configuration:
   • Go to Domain and OU filtering
   • Ensure all relevant OUs containing mailboxes are selected
6. Check attribute filtering:

   Get-ADSyncConnectorRunProfile -ConnectorName "domain.com" | Where-Object {$_.Name -eq "Full Synchronization"}

7. Force a full synchronization:

   Start-ADSyncSyncCycle -PolicyType Initial

8. Monitor synchronization status:

   Get-ADSyncSyncCycleResult

This advanced method ensures future mailboxes automatically receive the correct SMTP proxy addresses.

1. Open Exchange Admin Center and navigate to Mail flow → Email address policies
2. Create a new email address policy or edit the existing default policy
3. Click Add email address format and configure:
   • Email address type: SMTP
   • Email address format: @yourdomain.mail.onmicrosoft.com
   • Make this the reply address: Unchecked
4. Set the policy priority to ensure it applies correctly
5. Apply the policy to existing mailboxes using PowerShell:

   Get-Mailbox -ResultSize Unlimited | Update-EmailAddressPolicy

6. Verify the policy application:

   Get-EmailAddressPolicy | Get-Recipient -ResultSize Unlimited | Select-Object Name,EmailAddresses

7. Test with a new mailbox creation to ensure automatic proxy generation
8. Update the hybrid configuration to recognize the new routing domain:

   Set-HybridConfiguration -Domains @{Add="yourdomain.mail.onmicrosoft.com"}