Access Denied in New Outlook

We're users of Microsoft 365 Business Standard. At the time of writing (February 2025), Microsoft is stepping up its attempts to "encourage" users to migrate from the "classic Outlook" email client which is part of the Office desktop suite to the "new Outlook" client which is a Webview2 app, that is to say little more than Outlook Web Access with a new skin. One of our users found she was being forced to run New Outlook. Aside from the numerous features of Classic Outlook which are not replicated in New Outlook (such as PST files), we experienced the further problem of being unable to run New Outlook, instead receiving the message "This account has been blocked from accessing the new Outlook for Windows by your system administrator."

Well, I'm the system administrator and I don't recall blocking anything, but I found a resolution as follows:

Log into Microsoft 365 with an administrator account. Launch Windows Powershell as Administrator.

Install the MS-Exchange library to Powershell with "Install-Module -Name ExchangeOnlineManagement -Scope CurrentUser". You may find you already have it, or you may get a message about the software being in an unstrusted repositry, in which case respond with a "Y".

Bring your MS-Exchange Powershell library up to date with "Update-Module -Name ExchangeOnlineManagement". These commands take a while to run, by the way; don't lose patience.

Connect to your Microsoft 365 Exchange Server from Powershell: (search "Connect to Exchange Online PowerShell" for the MS learn page) You may have to authenticate to MS 365 and grant yourself permission to use this stuff.

In Powershell type:

Import-Module ExchangeOnlineManagement

Connect-ExchangeOnline -UserPrincipalName {admin email address]

(Replace [admin email address] with your Microsoft 365 administrator email and Ignore all the messages your receive.)

You can then examine a user's OneWinNativeOutlookEnabled policy setting with:

Get-CASMailbox -Identity [user email address] | Format-List Name,OneWinNativeOutlookEnabled

If this is set to False the user's access to New Outlook will be blocked. Set it to True as follows:

Set-CASMailbox -Identity [user email address] -OneWinNativeOutlookEnabled $true

You can then run New Outlook, toggle it off, answer Microsoft's questionnaire as to why you don't want to switch to it, and go back to using Classic Outlook.

And you can hide the toggle with a policy setting, but that's another story.

See https://techcommunity.microsoft.com/blog/outlook/things-to-know-about-the-new-outlook-for-windows/3383964 for some more information

This drove me crazy, and it was a simple fix, although by accident. Open edge, go to settings/history, and delete all time, every category. In my case the website was stored in a cookie, that cleared when I erased history. Cant promise it will work for you, but it did for me!

I have the Access denied problem but I notice it happens when I reply to an email.. That email gets stuck in Draft and Outlook says the email is not being sent. After Access denied and manage repair a couple of times I get back in only to get kicked out again. The next day when Iog in, I get an email message saying the email could not be sent.

What is MFA/2FA? How do you turn it off? I am having this "access denied" problem on a new computer (manufactured in 2024 and bought in 2025) using Windows 11 Home S. Outlook is called "new." I have never before heard of MFA and have no idea how to find it or turn it off. I need more help than this.

Just found your post, after days of "Access Denied" after a fleeting glance of the email folders deleting "Cookies and other site data" in History I thought had resolved this.
But now after 20 minutes the issue has returned.... :(
As @Cynthia Karban says Access Denied get triggered when sending an email...