10 Troubleshooting Methods to Fix Office 365 Mailbox Provisioning Stuck Preparing

IT admins usually get stuck in this “preparing” state for hours due to a breakdown in communication between the Entra ID and Exchange Online backend. Apart from this, there are other symptoms of a stuck mailbox as well (which we will cover in this blog post).

So, if you’re also looking for answers for queries like this, “I assigned an E3 license to a user in Microsoft 365, but their mailbox has been stuck on ‘preparing’ for 4 hours. What are the step-by-step troubleshooting methods to fix this?”

You must check out this comprehensive troubleshooting guide that shares 10 proven methods to fix Exchange Online Mailbox Provisioning issues.

What Causes Office 365 Mailbox Provisioning Stuck Preparing Issue?

Exchange Online mailbox provisioning issues happen due to a disruption in communication between Entra ID (Azure ID) and the Exchange Online Backend. Resulting in being stuck on the “We are preparing a mailbox for this user” message.

Apart from this message, there are some other common symptoms of a Stuck Mailbox as well. So, let’s check them out:

• The mailbox doesn’t show the assigned domain. But an internal placeholder address.
• The Exchange Admin Center does not feature the newly created user mailbox.
• The user will not be able to log in to Outlook on the Web (OWA). And encounters a 500 error or “Mailbox not found” prompt.

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How to Fix the Mailbox Not Provisioning Office 365 Preparing

Here are the 10 effective solutions to resolve the M365 Mailbox Provisioning Preparing issue:

 #1: Review the Exchange Online License

Reviewing the license and service health is the primary step to fix the Office 365 Mailbox provisioning stuck preparing error. It ensures whether the user is assigned a valid Exchange Online license (Business Basic, Standard, E3, or E5). Since allotting a license without an Exchange Online plan is a common mistake.

Here’s how to verify the Exchange Online License:

• Go to the M365 Admin Center.
• Right-click Active Users and select the user.
• Hit Licenses and apps to verify the Exchange Online License.

Also, make sure an Exchange Online Plan (Plan 1 or 2) is checked under the Apps section. And since Microsoft’s datacenter routing mandates a physical country assignment to legally provision the storage. So, ensure the user has a defined Usage Location, like the “United States”.

#2: Check Microsoft Service Health Dashboard

Check the Service health dashboard in the Microsoft 365 Admin Center to determine active advisories for Exchange Online. Because Microsoft sometimes experiences backend provisioning delays due to high latency. In simple words, during the month-end period, companies onboard new hires. Resulting in high latency in the provisioning queue.

In such scenarios, you have to wait until the active advisory clears up! To check the Service health, follow the given steps:

• Open Microsoft 365 Admin Center.
• Go to Health and right-click Service Health.

#3: Utilize the Native M365 Diagnostic Tool

Yes, Microsoft has a built-in “EXO Recipient Object Failures” automated diagnostic tool. It is engineered specifically to fix stuck mailboxes that force a background sync. It runs a backend script that reviews damaged AD objects. And attempt to push the Entra ID user object into the Exchange directory. Or say force mailbox sync Office 365.

Steps to run the Built-in-M365 Diagnostic Tool

• Go to Microsoft 365 Admin Center and log in.
• In the bottom right corner, right-click on the Help & support widget (question mark icon).
• In the search bar, copy-paste: Diag: EXO Recipient Object Failures or Mailbox not provisioning.
• Provide the target user’s UPN (User Principal Name) and run the test.

#4: Clear Attributes and Run Remote Mailbox (Hybrid Environment)

IT administrators cannot allot an Exchange Online License to an active AD user in a Hybrid environment. Because if you’re on-premise AD data in the msExchMailboxGuid or msExchHomeServerName attributes, Exchange Online presumes that the mailbox exists on-premise. Leading to an O365 assigned license but no mailbox.

Also, it is noteworthy for IT leaders that Exchange Online is engineered to prevent “split-brain” scenarios (users with a mailbox on-premise and in the cloud).

To fix this,

• Clear the attributes in the local Active Directory. To do so, you can use ADSI Edit or AD Users and Computers with Advanced Features on.
• Avoid assigning a cloud license to the user; run an Enable-RemoteMailbox -Identity “username” in the on-premises Exchange Management Shell.

Exchange Online will provision the cloud mailbox accurately once Azure AD Connect syncs this updated object state to the cloud.

You might not know, but you can force the Entra ID connect synchronization after fixing the attributes. You do not have to wait 30 minutes for automatic sync. Here are the steps to enforce Entra ID Connect synchronization:

• Go to the Entra ID Connect server and log in.
• Open PowerShell as an administrator.
• Execute the following PowerShell script:

Import-Module ADSync
Start-ADSyncSyncCycle -PolicyType Delta

#5: Fix Dehydrated Tenant

Dehydrated Tenant refers to the situation when multiple users are stuck in provisioning in a new Microsoft 365 tenant. Microsoft uses this tenant compression approach to save server resources. Sometimes, the automatic hydration process ceases.

Fortunately, you can write a PowerShell script to force provision a mailbox in Office 365 for a newly created user. So here’s how to fix the mailbox not provisioning Office 365 PowerShell

Steps to fix dehydrated tenants via PowerShell scripts:

• Open PowerShell as an administrator. Copy-paste the following PowerShell script to connect to Exchange Online:

Install-Module -Name ExchangeOnlineManagement -Force
Import-Module ExchangeOnlineManagement
Connect-ExchangeOnline

• Run the given script to review the hydration status of the tenant:

Get-OrganizationConfig | Format-List IsDehydrated

• If the value returns True, execute the following code:

Enable-OrganizationCustomization

Forcing stuck mailboxes provisioning can take up to 15 to 30 minutes. So, be patient after the above PowerShell Scripts.

#6: Cycle the License: The Trial License Trick

Do you know that you can also cycle the license to jumpstart the Office 365 provisioning mailbox? Yes, if the backend provisioning process is stuck on “We are preparing a mailbox for this user”, you can use the “trial license” trick.

Also, this quirky trick works like a charm on issues such as “payment method blocked during tenant creation”.

Steps to cycle the license:

• Begin by removing the current Exchange Online License (from the user and save changes).
• Go to Billing and select Purchase Services.
• Now, buy a free 30-day trial of a different license, such as Microsoft 365 Business Basic.
• Apply the newly purchased trial license to the user. Doing this will force Entra ID (Azure Active Directory) backend to re-assess the account. And triggering a fresh provisioning call.
• Lastly, the mailbox will be created. Simply, remove the trial license. And, reapply the original license.

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#7: Wipe out Duplicate Proxy Addresses or UPN

“Exchange admin center mailbox not showing up” or “Office 365 user has a license, but Exchange says preparing” can also occur if the proxy address assigned to the new user is already associated with any of the following:

• Deleted User
• Shared Mailbox
• Or a Contact Object

So, to ensure that Exchange Online will not stall the provisioning for hours, clear the duplicate proxy addresses or UPNs. To do so,

• Open the Exchange Online PowerShell and run the script cited below. Use your domain instead of “[email protected]”

Get-Recipient "[email protected]"

Note: If it returns with a different object, remove the issuing SMTP address from that object. Especially, before the new mailbox can be provisioned.

#8: Hard Delete & Recreate the User Account (For New Accounts Only)

Deleting & recreating the user account is the specific & fastest troubleshooting step to fix the “we are preparing a mailbox for this user” message. Including Exchange Admin Center, not showing the mailbox error.

Note: This troubleshooting method is for only new accounts with zero data (no OneDrive Files, no Teams history, no SharePoint activity). And stuck on the preparing message for over 24 hours.

• Open Microsoft Admin Center and click Active users.
• Select the user account and delete it.
• Now, go to Deleted Users and permanently delete the account. It will wipe out the UPN and email addresses from Entra ID. The directory will take up to 15 minutes to synchronize.
• Connect to Entra ID PowerShell and run the given script to ensure the account is permanently deleted. Replace “[email protected]” with your domain name:
  Remove-AzureADUser -ObjectId “[email protected]”
• Finally, create the user again and allot the license.

#9: Use SysTools Software to Fix Exchange Mailbox Provisioning

Mailbox provisioning issues often spike during bulk Office 365 tenant to tenant migration or from on-premise Exchange. Leaving IT admins with synchronization issues or native tool limitations. Imagine applying these troubleshooting methods to each user’s mailbox: true mailbox provisioning!

In such cases, MVPs and expert IT leaders always recommend the SysTools Office 365 to Office 365 Migration tool. It tackles these backend glitches effectively. And eliminates operational downtime.

• This ISO and Gartner-certified migration tool stricly comply to a Zero-Data Loss Policy. Promising migration of the entire Microsoft 365 workloads. It includes Emails, Contacts, Calendar, Tasks, and OneDrive Documents. Ensuring the exact attachment and folder hierarchies.
• It automates the user mapping process. It means no intense, consuming manual account creation errors. You can simply upload a CSV file to immediately map dozens of source users directly to their new destination addresses.
• It also provides an option to migrate priority accounts first concurrently. Including with amazing feature like Delta Migration that allows new email synchornization from the source tenant.

So, stop waiting 24 hours for Exchange mailbox provisioning. And hitt he download or purchase button  to migrate the complete mailbox data of 2 users for completely FREE!

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#10: Open Microsoft Support Ticket

Opening the Microsoft Support ticket in the Admin Center is the last resort to troubleshoot “Office 365 user has a license, but Exchange says preparing”.

Apply this proven method only if your mailbox has been frozen on “we are preparing a mailbox for this user” for over 24 hours. And, you cannot delete the existing data. It includes not running a hybrid environment. Because your mailbox is physically stuck in a failed database partition on Microsoft’s end.

• Simply, open a Microsoft Support ticket (Severity B) in the Admin Center.
• Explicitly explain that you have verified licensing, usage location, and have run the diagnostic tool. And, the object is stuck in provisioning. Request them to manually regenerate the mailbox GUID on the backend.

Conclusion

Exchange Mailbox provisioning stuck preparing can be a real productivity killer for organizations. Fortunately, by following the 10 robust methods outlined in this enterprise-grade guide, you can resolve the Office 365 mailbox not provisioning issue.

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FAQs (Frequently Asked Questions)

Q.1 How to fix mailbox not provisioning Office 365 stuck?
Fixing mailbox not provisioning Office 365 stuck can be done by reviewing the Exchange Online license, checking Microsoft Service Health, running the native diagnostic tool, or opening a Microsoft Support ticket.

Q.2 How long does it take for Office 365 mailbox to provision?

Usually, Microsoft 365 mailbox provisioning takes less than 30 minutes. But when the mailbox provisioning process gets stuck on the “we are preparing a mailbox for this user” screen, it will take 24 hours or more. To fix this, you have to take the proactive measures listed in this blog post.

Q.3 What is the difference between the provisioning process of a shared mailbox and a regular mailbox in Office 365?

A regular mailbox requires an active user with a paid Microsoft 365 license to create a mailbox. On the other hand, Shared Mailbox doesn’t mandate a separate paid license. It is provisioned as a native disabled user object.

Q.4 How to Force mailbox sync Office 365?

Here’s how you can force mailbox sync in Office 365:

• Run the built-in diagnostic tool,
• Wiping out duplicate active proxies
• Hard deleting & rebuilding the user mailbox
• Cycling the license
• Clear out the attributes in the local active directory and run Remote Mailbox.

Q.5 Exchange Admin Center mailbox not showing up?

If the Exchange Admin Center mailbox is not showing up, try verifying the license, checking the Service Health dashboard, using the native M365 diagnostic tool, or fixing a dehydrated tenant.