How to fix this.

Hi Dune and Kind,

I'm Sumit, here to help with your query on the Microsoft Q&A forums.

Apologies for any inconvenience you are experiencing. I am happy to help you today.

The message means you do not have a mailbox assigned. Please answer these questions:

1. Are you trying to send an email and you receive this message, or trying to log in?
2. Is this a Work or school account?

The IT Admin needs to correct this issue essentially if it is a Work or school account.

I look forward to your response and am eager to collaborate until we find a solution.

Hello @Dune and Kind

Good day! Thank you for reaching out to the Microsoft Q&A Forum. We’re happy to assist you. 

Based on the error message you provided, it appears that your account may not have a valid mailbox or the necessary license assigned.  

To begin troubleshooting, could you please confirm whether your account currently has a valid Microsoft 365 license that includes Exchange Online?  If you have admin privileges, you can verify this by signing in to the Microsoft 365 Admin Center. 

• Sign in to the Microsoft 365 Admin Center using an admin account. 
• Go to Users > Active Users. 
• Select your user account. 
• Click on Licenses and Apps. 
• Expand the Apps section and ensure that Exchange Online (Plan 1 or Plan 2) is selected. 

If the license is already assigned, please try unassigning it temporarily, then reassigning it. After that, wait for the synchronization period and try accessing Outlook Web App again. 

If you do not have admin access, kindly contact your organization’s administrator to perform the steps above on your behalf. 

You may also refer to this Microsoft Learn article for more details: Getting a mailbox not found error in Outlook on the web - Microsoft 365 admin | Microsoft Learn. 

If the issue still present, we recommend raising a support request via the Microsoft 365 admin center > Support > New Service Request. This will connect you with our support engineers who have the appropriate tools and permissions to investigate the issue in depth. 

You can refer to this article to check the way how to raise a ticket: Get support - Microsoft 365 admin | Microsoft Learn 

We look forward to your response. Your cooperation is greatly appreciated. 

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Hi @Dune and Kind

I hope this message finds you well.

I truly want to ensure your experience is as smooth as possible. If your problem is still ongoing or if you have faced any new difficulties, please share more details about the current steps you are stuck on. I am here and happy to provide further help whenever you need it.

While waiting for your response, I will continue tracking other customer questions on the forum to ensure everyone receives timely support. If you have additional questions, please feel free to tag my name in your reply so I can assist you promptly and more personally.

Looking forward to hearing from you soon.