Error message meaning I simply cannot open outlook mail

Hi @Tom Watson, 

Thank you for posting your question in the Microsoft Q&A forum.

Please understand that our forum is a public platform, and we will modify your question to cover your personal information including your organization domain name in the description. Please notice to hide these personal or organization information next time you post error or some information to protect personal data.        

Regarding that you can't log in to your email due to Error code 500.  To help narrow down the root cause. I’d like to ask some quick questions to better understand your situation and assist you effectively:  

Could you please confirm whether you have already checked with your administrator to ensure that your account still has an active license? 

If you have admin privileges, you can verify this by following these steps: 

1. Sign in to the Microsoft 365 Admin Center:  https://admin.microsoft.com/ 
2. Go to Users > Active users 
3. Select your user account 
4. Choose Licenses and apps 
5. Expand Apps and ensure that Exchange Online (Plan 1 or Plan 2) is enabled 

If the license is already assigned, try unchecking it, saving the changes, then reassigning it. Afterward, wait for the synchronization to complete and check if the issue is resolved. 

If you don't have Admin privilege, kindly contact your organization Admin to check above for you. 

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

If the issue still present, you may also (use admin account to) go to Office 365 Admin Center-> Support-> New Service Request. The support engineers there have the correct escalation channel, and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket: Ways to contact support for business products - Admin help. The engineers in related team have higher permission and resources than us to help check the root cause from background directly. 

Thank you again for your time and understanding. While my initial response may not resolve the issue immediately, I’d like to gather more details about your situation so I can assist you more effectively.         

I really appreciate your patience, and I’m here to help. Looking forward to your response.                      

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".                 

Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread