📋 Errors Covered in This Article

For each error we cover: the exact error message, what it means, common causes, resolution steps, and relevant PowerShell commands.

• UserAlreadyBeingMigratedException — User Already Being Migrated
• OnpremisesUserIsBeingMovedException — Existing Move Request Blocking Migration
• MigrationRecipientNotFoundException — Recipient Not Found
• TargetUserAlreadyHasPrimaryMailboxException — Target User Already Has a Primary Mailbox
• TargetDeliveryDomainMismatchPermanentException — Missing Target Delivery Domain
• NotAcceptedDomainException — Invalid SMTP Domain
• MailboxReplicationPermanentException — Archive Mailbox Already Exists
• CorruptFolderRule — Corrupt Inbox Rule
• MailboxExceedsTargetQuotaPermanentException — Mailbox Size Exceeds Target Quota
• QuotaExceededException — Folder Item Count Limit Reached
• MailboxDumpsterExceedsTargetQuotaPermanentException — Recoverable Items (Dumpster) Exceeds Quota

1. User Already Being Migrated

Exchange Online Error Message

What This Means

Resolution

1. 1
   Check for an existing move request in Exchange Online using Get-MoveRequest.
2. 2
   Remove the stale or completed move request with Remove-MoveRequest.
3. 3
   If no move request exists, verify whether the mailbox has already been successfully migrated to Exchange Online.

Connect-ExchangeOnline

# Check for existing move request
Get-MoveRequest -Identity user@domain.com

# Remove the stale request if found
Remove-MoveRequest -Identity user@domain.com

# Verify the mailbox exists in Exchange Online
Get-Mailbox -Identity user@domain.com | Select-Object DisplayName,RecipientTypeDetails

2. Existing Move Request Blocking Migration

Exchange Online Error Message

What This Means

Resolution

1. 1
   Connect to Exchange on-premises PowerShell and verify the existing move request.
2. 2
   Remove the completed or stuck move request.
3. 3
   Re-initiate the migration once confirmed clear.

# Run from Exchange On-Premises Management Shell

# Check for existing move request
Get-MoveRequest -Identity user@domain.com

# Remove the completed or stuck request
Remove-MoveRequest -Identity user@domain.com

# Confirm no move requests remain
Get-MoveRequest -Identity user@domain.com

3. Recipient Not Found

Exchange Online Error Message

What This Means

Common Causes

• User account deleted from Active Directory
• User account disabled before migration completed
• Mailbox object missing or corrupt
• Azure AD synchronisation issue — user not synced to Exchange Online

Resolution

1. 1
   Verify the mailbox exists in Exchange on-premises using Get-Recipient.
2. 2
   Confirm the user account is active and enabled in Active Directory.
3. 3
   Check Azure AD Connect synchronisation status and force a delta sync if needed.
4. 4
   Restart the migration once the recipient object is confirmed and synchronised.

# Verify recipient exists on-premises
Get-Recipient user@domain.com

# Check mailbox object
Get-Mailbox -Identity user@domain.com

# Force Azure AD Connect delta sync (run on the AD Connect server)
Start-ADSyncSyncCycle -PolicyType Delta

4. Target User Already Has a Primary Mailbox

Exchange Online Error Message

What This Means

Common Causes

• User was already migrated in a previous batch
• Duplicate mailbox configuration — both on-premises and cloud mailboxes exist
• A cloud mailbox was created manually before migration was initiated

Resolution

1. 1
   Verify the mailbox location and recipient type to confirm where the primary mailbox currently exists.
2. 2
   If a duplicate mailbox exists in Exchange Online, disable the on-premises mailbox.
3. 3
   Enable the remote mailbox on-premises to properly represent the cloud mailbox.
4. 4
   Initiate a new New-MoveRequest from the on-premises Exchange to cloud.

# Check recipient type and location
Get-Recipient -Identity user@domain.com | Select-Object DisplayName,RecipientType,RecipientTypeDetails

# Verify if mailbox exists in Exchange Online
Get-Mailbox -Identity user@domain.com | Select-Object DisplayName,RecipientTypeDetails,ExchangeGuid

# Disable on-premises mailbox if duplicate exists
Disable-Mailbox -Identity user@domain.com

# Enable as remote mailbox pointing to Exchange Online
Enable-RemoteMailbox -Identity user@domain.com -RemoteRoutingAddress user@tenant.mail.onmicrosoft.com

5. Missing Target Delivery Domain

Exchange Online Error Message

What This Means

Resolution

1. 1
   Open Exchange on-premises Admin Center (ECP) and verify that email address policies are applied to the mailbox.
2. 2
   Enable automatic address updates if the policy is not stamping the mail.onmicrosoft.com routing address.
3. 3
   Manually add the routing address if email address policies are not covering this mailbox.
4. 4
   Restart the migration once the routing address appears in the proxy addresses.

# Check existing proxy addresses
Get-Mailbox -Identity user@domain.com | Select-Object -ExpandProperty EmailAddresses

# Manually add the missing routing address
Set-Mailbox -Identity user@domain.com -EmailAddresses @{Add="user@tenant.mail.onmicrosoft.com"}

# Force email address policy update on the mailbox
Set-Mailbox -Identity user@domain.com -EmailAddressPolicyEnabled $true

# Verify the address was added
Get-Mailbox -Identity user@domain.com | Select-Object -ExpandProperty EmailAddresses

6. Invalid SMTP Domain

Exchange Online Error Message

What This Means

Resolution

1. 1
   Identify all invalid proxy addresses on the mailbox (e.g. addresses using @localhost, @localdomain, or other non-routable domains).
2. 2
   Remove the invalid SMTP address from the mailbox.
3. 3
   Restart the migration once the invalid address is removed.

# List all proxy addresses to identify invalid ones
Get-Mailbox -Identity user@domain.com | Select-Object -ExpandProperty EmailAddresses

# Remove the specific invalid SMTP address
Set-Mailbox -Identity user@domain.com -EmailAddresses @{Remove="user@localhost"}

# Verify removal
Get-Mailbox -Identity user@domain.com | Select-Object -ExpandProperty EmailAddresses

7. Archive Mailbox Already Exists

Exchange Online Error Message

What This Means

Resolution

1. 1
   Enable the remote archive mailbox on-premises to align the hybrid objects.
2. 2
   Create the move request using the -PrimaryOnly parameter to migrate only the primary mailbox and avoid the archive conflict.

# Enable remote archive mailbox on-premises
Enable-RemoteMailbox "user@domain.com" -Archive

# Create move request with -PrimaryOnly to skip archive migration
New-MoveRequest -Identity "user@domain.com" -Remote -TargetDeliveryDomain tenant.mail.onmicrosoft.com -PrimaryOnly

# Monitor the move request
Get-MoveRequest -Identity "user@domain.com" | Select-Object Status,PercentComplete,Message

8. Corrupt Inbox Rule

Exchange Online Error Message

What This Means

Resolution

1. 1
   Review all Inbox rules on the mailbox to identify any that are invalid, circular, or reference missing folders.
2. 2
   Remove the problematic rule by name or identity.
3. 3
   Resume the migration once the corrupt rule is removed.

Connect-ExchangeOnline

# List all inbox rules for the mailbox
Get-InboxRule -Mailbox user@domain.com | Select-Object Name,Enabled,Description

# Remove the problematic rule by name
Remove-InboxRule -Mailbox user@domain.com -Identity "Rule Name" -Confirm:$false

# Verify remaining rules
Get-InboxRule -Mailbox user@domain.com | Select-Object Name,Enabled

9. Mailbox Size Exceeds Target Quota

Exchange Online Error Message

What This Means

Exchange Online Mailbox Quotas by Licence

Resolution

• Upgrade the user’s Exchange Online licence to Plan 2 or an E3/E5 plan
• Enable Online Archive (In-Place Archive) to offload older content
• Move historical mail to archive before migration
• Export older data to PST and import post-migration
• Clean Deleted Items and Junk Email folders before migrating

# Check mailbox size on-premises
Get-MailboxStatistics -Identity user@domain.com | Select-Object DisplayName,TotalItemSize,ItemCount

# Check mailbox size in Exchange Online
Connect-ExchangeOnline
Get-MailboxStatistics -Identity user@domain.com | Select-Object DisplayName,TotalItemSize,ItemCount

# Enable Online Archive in Exchange Online
Enable-Mailbox -Identity user@domain.com -Archive

10. Folder Item Count Limit Reached

Exchange Online Error Message

What This Means

Resolution

1. 1
   Identify folders with the highest item count using Get-MailboxFolderStatistics, sorted descending.
2. 2
   Reduce the item count by deleting unnecessary emails in the over-limit folder.
3. 3
   Split the folder contents into subfolders to distribute items across multiple containers.
4. 4
   Archive older emails or export to PST and remove from the mailbox.
5. 5
   Restart the migration after cleanup.

# Identify folders sorted by item count (highest first)
Get-MailboxFolderStatistics -Identity user@domain.com |
  Select-Object Name,ItemsInFolder,FolderSize |
  Sort-Object ItemsInFolder -Descending |
  Format-Table -AutoSize

# Find folders exceeding 1 million items
Get-MailboxFolderStatistics -Identity user@domain.com |
  Where-Object { $_.ItemsInFolder -gt 1000000 } |
  Select-Object Name,ItemsInFolder,FolderSize

11. Recoverable Items (Dumpster) Exceeds Quota

Exchange Online Error Message

What This Means

Common Causes

• Litigation Hold enabled — retains all deleted items indefinitely
• In-Place Hold applied — retains items matching search criteria
• Single Item Recovery enabled — retains deleted items for the recovery period
• Large Purges folder — items purged by the user but retained by holds
• Large Versions folder — multiple versions of items stored by holds

Diagnose

Connect-ExchangeOnline

# Verify total deleted item size and count
Get-MailboxStatistics user@domain.com | Format-List TotalDeletedItemSize,ItemCount

# Verify hold settings on the mailbox
Get-Mailbox user@domain.com |
  Format-List LitigationHoldEnabled,InPlaceHolds,SingleItemRecoveryEnabled

# Verify Recoverable Items folder breakdown
Get-MailboxFolderStatistics -Identity user@domain.com -FolderScope RecoverableItems |
  Select-Object Name,FolderAndSubfolderSize,ItemsInFolder

Resolution

1. 1
   Remove mailbox holds — ensure legal and compliance teams have approved this step before removing any holds.
2. 2
   Disable Single Item Recovery if not required.
3. 3
   Reset retention periods where permitted by policy.
4. 4
   Run the Managed Folder Assistant to process retention policies and allow the Recoverable Items folder to reduce in size.
5. 5
   Allow time for the dumpster to drain, then restart the migration.

Connect-ExchangeOnline

# Remove Litigation Hold (ensure legal approval first)
Set-Mailbox -Identity user@domain.com -LitigationHoldEnabled $false

# Disable Single Item Recovery
Set-Mailbox -Identity user@domain.com -SingleItemRecoveryEnabled $false -RetainDeletedItemsFor 14

# Run Managed Folder Assistant to process retention immediately
Start-ManagedFolderAssistant user@domain.com

# Confirm Recoverable Items size after processing
Get-MailboxFolderStatistics -Identity user@domain.com -FolderScope RecoverableItems |
  Select-Object Name,FolderAndSubfolderSize,ItemsInFolder

Conclusion

Exchange Hybrid Remote Move migrations from Exchange Server 2016, Exchange Server 2019, and Exchange Server Subscription Edition (SE) to Exchange Online are generally reliable, but administrators may occasionally encounter mailbox, directory, archive, quota, or synchronisation-related errors.

Understanding the root cause of these errors is often the key to resolving them quickly and avoiding unnecessary troubleshooting. By performing proper pre-migration checks and following the resolutions in this article, most migration failures can be identified and corrected before they significantly impact migration timelines.

• Always run Get-MoveRequest before initiating migration to check for existing or stale requests
• Verify all proxy addresses include the @tenant.mail.onmicrosoft.com routing address
• Check mailbox size against the target licence quota and apply a buffer before migrating
• Review Inbox rules for circular or invalid conditions that corrupt folder rules
• Check holds and Recoverable Items size for large mailboxes before initiating migration
• Run Get-MailboxFolderStatistics on large mailboxes to identify folders approaching the 1M item limit