I cannot access my Office 365 mail from Outlook 365 client any more.

Hi Betty,

My Apologies for not responding sooner. I was confused about this thread not showing up where I originally posted on someone else's thread.

I see that you forked this and someone else forked the other thread. I was responding to the other thread.

Thank you for suggesting to check the Fiddler logs. This lead me in the proper direction to solve my problem.

Here was my closing post from the other thread. I am also posting here in case it may help someone else.

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Hi Gabriel,

After some work I was able to get things working.

Thank you for your assistance.

My biggest concern is what triggered this situation.

Was it me turning on the Modern Authentication on in my tenant?

Some notes for anyone that runs across this thread:

I will consider the view that you want to turn on Multi Factor Authentication (MFA) in your tenant.

I do not know if there is a document I did not follow but I went into my security settings in my tenant and enable 2FA for my admin account.

I believe I started having issues with my user account after that. I have NO confirmation from MS about this unless they post a reply about this.

What I do know is that I could not log into my user account and Fiddler showed that Oauth was not turned on. What triggered this I do not know.

Ignoring all the things that did not work. 

Here are my notes on what worked.

NOTE: Read the 2 pages that I have linked. The documentation is good but you have to read it.

I had MFA Turned on in my admin account and my user account.

I then had to connect to Exchange Online using the instructions in:

Connect to Exchange Online PowerShell using multi-factor authentication

https://docs.microsoft.com/en-us/powershell/exchange/exchange-online/connect-to-exchange-online-powershell/mfa-connect-to-exchange-online-powershell?view=exchange-ps

 MFA requires you to install the Exchange Online Remote PowerShell Module, and use the Connect-EXOPSSession cmdlet to connect.

READ THE NOTE at the top of the article. Important information especially if you are a Delegated Administrator.

The Exchange Online Remote PowerShell Module needs to be installed on your computer.

Note: You have to use Edge or Internet Explorer to install Exchange Online Remote PowerShell Module.

The section on the Windows Remote Management did not work for me. Not sure why, does not seem to matter.

Once you are able to connect to Exchange you can follow the instructions on the next link to enable Oauth for the clients to use.

Enable modern authentication in Exchange Online

https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/enable-or-disable-modern-authentication-in-exchange-online

After you have made all these changes, I made sure I logged out of the account where I could and reboot.

When I logged backed in, my OneDrive connected without issue. I logged into Word and had to confirm the login. To confirm the login I was using the Microsoft Authenticator app on my phone. Very convenient, it just pops ups and asks you to approve or decline. Signed into Outlook without any issue.

Thanks again to Gabriel for your help.

I hope the information helps,