I can't launch new Outlook

Hi @Mark Helman, 

I hope you are doing well. 

The issue you are experiencing could occur when there are too many sessions open in your user mailbox. To resolve the issue, I would recommend signing out of your Microsoft account on every device. To do this, please follow these steps: 

1. Sign out everywhere 

• Sign out of Outlook on every device (desktop, web, mobile). 
• Close all browsers and apps. 
• Wait 15–30 minutes, this allows Exchange to drop all stale sessions and object handles. 

2. Access via Outlook Web (OWA) 

• Log into Outlook on the web (if it still errors, skip to Step 3) 
• Go to Settings > Accounts > Shared with me. 
• Remove any shared or delegated mailboxes. 

• Delete or archive unused folders to keep total folder count below ~500, including subfolders. 
• Large numbers of folders and shared mailboxes are the triggers for this error. 

3. Contact your IT admin to reset your mailbox object handles 

If the error still appears even after cleanup, please reach out to your organization's IT admin to clear the remaining objects that are stuck on the server side by running this PowerShell command: 

New-MailboxRepairRequest -Mailbox "******@domain.com" -CorruptionType FolderView 

This forces Exchange to rebuild the folder view cache and drop all stuck object handles. 

For detailed guidance to install and connect to Exchange Online, your IT admin may refer to this document: Connect to Exchange Online PowerShell | Microsoft Learn

I hope this information is helpful. Please follow the steps above and let me know if it works for you. If not, we can continue working together to resolve the issue.  

Thank you for your patience and understanding. If you have any questions or need additional assistance, please don’t hesitate to reach out so I can continue to support you. If you found the response useful, please consider marking it as accepted, as this may help other community members who are looking for similar guidance.  

I look forward to continuing the conversation.

If you have extra questions about this answer, please click "Comment". 

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The error Microsoft.Exchange.Data.Storage.TooManyObjectsOpenedException indicates a server-side limit on the number of sessions/objects opened for the mailbox, not a storage (quota) issue. Increasing mailbox storage does not resolve this.

In Exchange, the Store limits the number of non-MAPI sessions per user (for POP, IMAP, OWA, ActiveSync, EWS, and related services). By default:

• Sessions per user are limited to 32
• Service sessions per user are limited to 16

When these limits are exceeded, the server throws TooManyObjectsOpenedException and blocks additional connections, which can prevent new Outlook from launching successfully against that mailbox.

Actions to take:

1. Fully sign out of all clients using this mailbox
   • Sign out from Outlook on the Web (browser), new Outlook for Windows, classic Outlook, mobile mail apps, and any other mail clients using the same account.
   • Close all those apps and browsers after signing out.
2. Use the global sign-out option
   • Use the “Sign out of everywhere” page referenced in the context to invalidate existing sessions: https://go.microsoft.com/fwlink/?linkid=2253707
   • This helps clear stale sessions that may be keeping the session count above the limit.
3. Wait for server-side cleanup
   • Even after signing out everywhere, it can take some time for all sessions to be released on the server side.
   • In a similar reported case, access was restored only after several hours (up to about 3 days in the worst case), even though the sign-out was done earlier.
4. Try launching new Outlook again
   • After some time has passed and sessions have cleared, start new Outlook for Windows again and sign in.
5. If the issue persists in an Exchange Server (on-premises) environment
   • An Exchange administrator can review the mailbox’s session usage and, if appropriate, adjust the relevant limits on the server side according to organizational policy.

Because this is a server-side session limit, local Outlook profile, cache, or storage changes on the client will not resolve it; the key is reducing/clearing concurrent sessions and allowing the server to drop old ones.

References:

• You cannot connect to an Exchange Server 2010 mailbox from Outlook Web App
• Outlook.com address returning TooManyObjectsOpenedError - Microsoft Q&A