Suddenly, as of today, Outlook Classic won't start because MSO.dll is incompatible with Microsoft Outlook.

I have done this more than 10 times trying to get one of two Gmail accounts working and it keeps giving me the IMAP error. I can get one to work but not the other one. I have uninstalled Microsoft 365 and reinstalled. I have removed the credentials. I have tried everything but it just doesn't work. It normally starts with an error when the loop back (local host message) says you can close this after enabling the Gmail access. I have also tried turning off Windows security, the firewall everything. I have spent 12 hours doing this over and over again. What can I change or load to get this working. I cannot use the new version because it does not work with our business processes. BTW one account words but the other one does not.

how about old history?

Dear Rafael Bernal

Welcome to the Microsoft Community.

Sorry to hear that you are having problems with Outlook not starting and adding email accounts being limited.

I would like to get more detailed information from you:

1. What version of Classic Outlook do you mean? It should be noted that Outlook in Microsoft Office 365 is also called Classic Outlook. Knowing the specific version information is very important for us to determine compatibility issues and find appropriate solutions. You can check the software's about information or the properties of the installation file to determine the version number.
2. Which account domains are currently configured in the software that reports the error? This will help us understand whether there is a compatibility issue between a specific account type or domain name and the software. In addition, if possible, can you share a detailed error screenshot? This way we can see the error information more intuitively, such as the error code, the complete error message content, etc., so that we can troubleshoot the problem more accurately.

In view of the MSO.dll file incompatibility issue you mentioned, the following is a general solution.

You can use the official uninstall tool to try to completely uninstall the software. For Microsoft Office products, Microsoft has a dedicated uninstall support tool that can help you completely remove the residual files and registry entries of the software. After the uninstallation is complete, re-download the installation package and install Outlook under the account you purchased the software.

1. Download and install the Microsoft 365 Uninstallation Support Tool: Uninstall Office from a PC - Microsoft Support
2. Please go to Microsoft account | Services & subscriptions and log in with the Microsoft account associated with the Office product.
3. Under 'Services & Subscriptions', find the product you need to install and select 'Install'.
4. Depending on your browser, select Run (in Microsoft Edge or Internet Explorer), Setup (in Chrome), or Save File (in Firefox).
5. After the installation is complete, search for any Office component in the Start menu or in the search bar at the bottom of the computer, and open it for activation.
6. For more information, see Download and install or reinstall Microsoft 365 or Office 2021 on a PC or Mac - Microsoft Support

We look forward to hearing from you. With your help and more detailed information, we can work together to find a solution. Thank you for your cooperation.

Thank you for your patience and understanding.

Forry.F - MSFT | Microsoft Community Support Specialist