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Quick Answer

Restart Outlook, then manually add the mailbox by going to File > Account Settings > Change > More Settings > Advanced and clicking Add.

How to Fix "Mailbox Not Found" Error in Outlook ​

Applies to: Outlook for Windows, Outlook for Mac, Outlook on the Web, Microsoft 365
Article Type: Troubleshooting
Last Updated: 2026-07-10

Summary ​

When you try to open a shared mailbox (like [email protected]) or a former employee's inbox that was assigned to you, Outlook may throw a "Mailbox Not Found" or "Cannot expand the folder" error. This guide walks you through forcing Outlook to sync the permissions so you can access the mail.

Symptoms ​

  • You click on a shared mailbox on the left side of Outlook, and a pop-up says Cannot expand the folder. The set of folders cannot be opened.
  • Outlook on the Web shows 500 Something went wrong: Mailbox Not Found
  • You know IT granted you access to the mailbox 5 minutes ago, but it still isn't working

Prerequisites ​

  • You must be signed in to your primary work email account
  • You must have been explicitly granted permission to access the target mailbox by your IT team or a manager

Instructions ​

1. Wait for Microsoft 365 to Sync (The Most Common Fix) ​

If IT has only recently granted you access to a shared mailbox, it does not appear instantly.

Microsoft 365's servers (Azure AD and Exchange Online) take time to synchronize permissions globally.

  • Typical wait time: 15 to 30 minutes.
  • Maximum wait time: Up to 24 hours (rare, but it happens).

If it has been less than an hour since you got access, wait and try again later.

2. Restart the Outlook Desktop App ​

If you've waited an hour and the mailbox still shows an error, your Outlook desktop app might be holding onto an old, cached version of your permissions.

  1. Close Outlook completely (click the X in the top right corner).
  2. Wait 10 seconds.
  3. Reopen Outlook.
  4. Check the bottom right corner of the Outlook window. It will say Connected to Microsoft Exchange.
  5. Try expanding the shared mailbox folder again.

3. Open the Mailbox in Outlook on the Web (The Override) ​

If the desktop app is stubborn, you can bypass it entirely by using the web version of Outlook. This proves whether the issue is with your computer or your actual account permissions.

  1. Go to https://outlook.office.com and sign in with your work email.
  2. Click your Profile Picture (or initials) in the top right corner.
  3. Select Open another mailbox.
  4. Type the name or email address of the shared mailbox (e.g., "Sales" or [email protected]).
  5. Select it from the list and click Open.

What happens next?

  • If it opens in a new tab: Your permissions are correct! The issue is your desktop app lagging behind. You can work from the web browser for now, and the desktop app catches up within a day.
  • If it says "500 Something went wrong" or "Access Denied": Your permissions are not set correctly on the server. You need to contact IT (see Step 5).

4. Manually Add the Mailbox in Outlook Desktop ​

Sometimes the "automapping" feature (which makes the mailbox magically appear on the left) breaks. You can manually force it to appear.

In Classic Outlook for Windows:

  1. Click File > Info > Account Settings > Account Settings.
  2. Click your email address, then click Change.
  3. Click More Settings.
  4. Go to the Advanced tab.
  5. Under "Mailboxes", click Add.
  6. Type the name of the shared mailbox and click OK.
  7. Click Apply, then OK, then Next, then Finish.

In New Outlook for Windows / Mac:

  1. Right-click your primary email address on the left sidebar.
  2. Select Add shared folder or mailbox.
  3. Type the email address of the shared mailbox and click Add.

5. What to Tell IT if It Still Fails ​

If you tried Step 3 (Outlook on the Web) and got an "Access Denied" or "Mailbox Not Found" error, the permissions were not applied correctly.

Submit a ticket to IT and say:

"I cannot access the shared mailbox [[email protected]]. I tried opening it via 'Open another mailbox' in Outlook on the Web, and it still gave an Access Denied/Not Found error. Can you please verify that my account has 'Full Access' permissions?"

Troubleshooting ​

Symptom / ErrorPotential CauseSolution
"Cannot expand the folder"Permissions haven't synced yet, or Outlook needs a restartWait 60 minutes, then restart the Outlook desktop app
Mailbox name shows up, but no emails load insideSync slider issue or network lagManually update the folder (Send/Receive tab > Update Folder)
"500 Something went wrong" in WebmailPermissions missing or mailbox deletedContact IT to verify the mailbox still exists and you have "Full Access"
You can read emails but can't reply from the mailboxMissing "Send As" permissionsIT must explicitly grant "Send As" permissions separately from "Full Access"