Outlook Classic Authentication

Question

I manage several users who use Outlook Classic as their primary email app. Today a single user noticed her calendar invites weren't going through. She had an email stuck in her outbox , and when I tried to clear it it said she didn't have permission to delete. I was able to delete it in safe mode, but new messages still get stuck and she is not receiving incoming messages despite the bottom ribbon saying "connected to MS Exchange". I tried making a new profile but the new profile gets hung on the "loading profile" step, same thing if i try to rename the existing OST to force a new one. The new profile's OST file is blank so I don't think it actually pulled any data. She has another user's mailbox added in the form of a second profile, and that one works just fine. New Outlook, mobile, and web worked fine so I assumed it must be an issue with either her cached credentials or something local, but here's the weird thing.

I made a new profile using her mailbox on my pc to test, and that failed. Creating the profile through the control panel gave me a notification that an encrypted connected could not be secured. Creating it through the profile selection menu worked including prompting the MS sign in popup, but trying to open the profile in or out of safe mode triggers the following error before crashing outlook; "Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The attempt to log on to Microsoft Exchange has failed". This is a single user in our organization who is unable to sign into multiple PCs via outlook classic, and does not appear to be tied to her credentials based on them working for the other clients.

Any Ideas? No major changes have been made on the back end that would impact authentication/sign ins.

Answer 1

Update*

User's mailbox is now accessible on both her device and the test device despite no action being taken. As far as we can tell this fixed itself which is a bit unnerving since it could always happen again. Chalking it up to a glitch with the legacy authentication Classic Outlook uses for now, thank you.

Answer 2

Hi @Joseph Perrotta, 

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

The fact that the issue follows the user's mailbox across multiple PCs, while Outlook on the web, mobile, and New Outlook continue to work, is certainly interesting and may help narrow down the cause.

A few questions that may help scope this down further: 

• Is the affected mailbox an Exchange Online mailbox, on-premises Exchange mailbox, or a hybrid environment? 
• On the 2nd PC where you tested, had that affected mailbox ever been signed into previously using Outlook Classic, or was this the first time it was added? 

Also, could you verify whether Work Offline mode is enabled in Outlook Classic? 

1. Open Outlook Classic. 
2. Go to the Send/Receive tab. 
3. Check whether Work Offline is highlighted. 
4. If it is, select it once to switch Outlook back online. 

If the 2nd PC had never previously signed in with the affected mailbox, you could also try clearing any saved Office/Outlook credentials: 

1. Close Outlook. 
2. Open Credential Manager in Windows. 
3. Check both Windows Credentials and Generic Credentials. 
4. Remove any entries related to Microsoft 365, Outlook, Office, ADAL, AzureAD, or Exchange. 
5. Restart the PC and try adding the mailbox again. 

If the issue persists after these checks, especially since: 

Outlook on the web works, 

Mobile works, 

New Outlook works, 

The problem appears to follow the mailbox across multiple PCs, 

then it may require deeper mailbox-level diagnostics. In that case, I would recommend that your Microsoft 365 Global Administrator create a support request with Microsoft so the support team can review the mailbox configuration and collect additional logs. 

For detailed instructions on how to get support, please refer to Get support - Microsoft 365 admin.     

Thank you again for your time and understanding. I really appreciate your patience, and I'm here to help. Looking forward to your response. 

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Answer 3

Because the failure follows the mailbox across multiple PCs, while new Outlook, mobile, and web still work, the most likely supported path is to treat classic Outlook profile/configuration as the problem first and remove unsupported profile layout if present.

1. Check whether the user’s mailbox and the other user’s mailbox were added as two Exchange accounts in the same classic Outlook profile.
   • That configuration is not supported.
   • If both mailboxes are added as separate Exchange accounts, remove the second Exchange account from the profile.
   • Then add the other mailbox as an additional mailbox instead:
     • File > Info > Account Settings > Account Settings
     • Select the primary account > Change > More Settings
     • Advanced tab > Add
     • Enter the mailbox name > OK > Next > Finish
   • Expected result: the additional mailbox appears on the Advanced tab, and the unsupported multi-Exchange-account condition is removed.
2. Create a completely new classic Outlook profile and test only the primary mailbox first.
   • Close Outlook.
   • Open Control Panel > Mail > Show Profiles.
   • Select Add, enter a new profile name, and complete setup with only the affected user’s primary mailbox.
   • Set Prompt for a profile to be used.
   • Start Outlook and choose the new profile.
   • If the new profile works, the old profile was corrupt.
3. If Outlook opens only in safe mode or add-ins are suspected, test safe mode again to isolate add-in/profile behavior.
   • Press the Windows key.
   • Type outlook.exe /safe and press Enter.
   • If sending works only in safe mode, create a new Outlook profile again and keep testing with only the primary mailbox.
4. Repair the Outlook profile or Office installation.
   • A damaged Outlook program or profile can block send/receive and startup.
   • Run Office repair first. If needed, repair the Outlook profile afterward.
5. Repair Outlook data files.
   • Since mail was stuck in Outbox and the OST rebuild did not complete correctly, repair the Outlook data files with the Inbox Repair Tool (scanpst.exe).
   • This is appropriate when Outlook reports folder or information store errors.
6. If the user recently changed password, update it in classic Outlook.
   • A password change on the account side must also be reflected in classic Outlook.
7. If antivirus email scanning is enabled, test with that product’s email scanning checked.
   • Some antivirus products can prevent sending or receiving after detecting a problem.

Given the exact error pattern described — classic Outlook failing for this mailbox on multiple PCs, including profile creation/open failures — the highest-value checks are:

• remove any unsupported second Exchange account configuration,
• create a brand-new profile with only the primary mailbox,
• repair Outlook data files and Office,
• then retest before adding any shared/additional mailbox back.

If the mailbox still fails only in classic Outlook after those steps, while web/mobile/new Outlook continue to work, that indicates the issue is outside normal self-service profile repair and needs escalation through Microsoft 365 admin support.

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References:

• Issues that can occur when you add multiple Exchange accounts to the same Outlook profile
• Classic Outlook not responding, stuck at "Processing," stopped working, or freezes
• I can't send or receive messages in Outlook
• Outlook for Windows not responding, hangs, freezes, or stops working
• Cannot log in Outlook on Window 11 anymore Error Massages - Microsoft Q&A
• Error 7q6cd code - Microsoft Q&A