Cannot launch Outlook 365 offline - "the set of folders cannot be opened"

Occasional Contributor

Windows 10 21H2 (OS Build 19044.1706)

Office 365 2204 (Build 15128.20178)


I'm unable to launch Outlook 365 offline. I get the error: "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." Once Outlook has started and successfully connected, I can then go offline, but if I am not online during launch I can never get in.

I've tried:
- starting in safe mode

- sfc /scannow

- dism /online /cleanup-image /checkhealth

- creating a new profile

- creating a new C:\Users\...\appdata\local\Microsoft\Outlook folder

- toggling cached exchange mode on and off

- toggling "turn on shared calendar improvements"

- disabling "download shared folders" in advanced account settings

- disabling "favorites" in the folder pane

- creating a registry REG_DWORD key HKCU\Software\Microsoft\Office\16.0\Outlook\AutoDiscover\ExcludeScpLookup with Value 1


None of these change the behavior.


Any ideas?

5 Replies
best response confirmed by ajkessel (Occasional Contributor)
Found a fix! Set created key in HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover

DWORD ExcludeExplicitO365Endpoint = 1

Now I can start offline. As a bonus, Outlook also launches much faster.

@ajkessel Hello, ajkessel, if your Outlook shows "the set of folders cannot be opened" issue, the first thing you should try is to open it in a safe mode.


To do it correctly, press the combination of keys ⊞ Win+R, in the Run menu, input Outlook.exe /safe, and confirm it by hitting OK.


Now you have to check if the problem remains. If not, this means the reason is in add-ins, which you can disable using these instructions:


Go to Outlook - File - Options - Add-ins. After it, hit on Manage and Go. Here you can choose the add-ins to disable and find out which of them causes the issue. Here you can find a list of other possible solutions to your problem:


Also, I recommend using this Office 365 backup software to make sure you can recover your organization's data whenever you need to.

As noted above, I already tried safe mode. I also tried toggling all the add-ins. None of that helped, but adding the registry key mentioned in my follow-up post eliminated the problem.
Bonus Tips - always have a backup of email data on cloud or external storage media.
I would suggest you to try Email backup solution - third party backup tools can be used but first try the demo prior to go with premium, check the strength and effectiveness and authenticity.
Thanks. This works for my users. I was not able to figure it out. Notice it affects more older version of outlook. Current ones don't have such Autodiscover issues.