Just what exactly are you talking about? Perhaps you can point to a link that explains it. As far as I know there is nothing but some changes to DKIM, DMARC and SPF requirements at Google and Yahoo in the wind and they have nothing to do with email clients, only servers.

Sorry, I meant Microsoft, not Gmail. Below is the email that MS sent me:

Update your sign-in technology before September 16th, 2024 to maintain email access.

The safety and security of your information is top priority for Microsoft. To help keep your account secure, Microsoft will no longer support the use of third-party email and calendar apps which ask you to sign in with only your Microsoft Account username and password. To keep you safe you will need to use a mail or calendar app which supports Microsoft’s modern authentication methods. If you do not act, your third-party email apps will no longer be able to access your Outlook.com, Hotmail or Live.com email address on September 16th.

What do you need to do?

If you are receiving this email, you are currently using an email or calendar app that uses a less secure authentication method to connect to your Outlook.com email account. You will need to upgrade your third-party mail and calendar app to a version which supports modern authentication methods.

Microsoft provides free versions of Outlook for your PC, Mac, iOS, and Android devices which can be easily downloaded and connect to your email account. Using an updated version of an Outlook application will ensure you are connecting in the most secure way.

How can you set up your Gmail, Apple Mail, or other third-party mail application?

Various non-Microsoft applications will have their own steps for connecting to your Outlook.com email account using modern authentication methods. See our help article - Modern Authentication Methods now needed to continue syncing Outlook Email in non-Microsoft email apps. However, you may need to contact the creators of those applications to provide you with instructions. In many cases, simply removing and re-adding your account with the latest version of that application will configure it to use modern authentication methods.

This is a mandatory service communication. To set your contact preferences or to unsubscribe from other communications, visit the Promotional Communications Manager. Privacy Statement.

Microsoft Corporation One Microsoft Way Redmond, WA 98052

I got that e-mail over a month ago. At the time I was using Thunderbird 45 which was too old to use "modern authentication methods" but Thunderbird 115 works fine with it. It's actually the same OAuth2 that GMail has used for a while.

Hi - Thanks for replying! Thunderbird Desktop Version 128.1.0esr | Released August 6, 2024 I have no clue why my version is way different from yours. I can say that I have to click on retry or cancel after every attempt to refresh Outlook.com using Thunderbird. Then repeat, repeat ... until I cancel. Sometimes it works, like it seems to say "get ready, we're cutting you off." They're using 2-factor verification whenever they want so this may be their hangup. They may want Mozilla to verify our identity from time to time.

Thanks for helping and good luck!

In future , please do more research than posting that the sky is falling. I recall that Microsoft announcement and they nicely included a link on how to set Thunderbird will full instructions. Many have posted here and the solution has worked for them.

 https://support.microsoft.com/en-us/office/modern-authentication-methods-now-needed-to-continue-syncing-outlook-email-in-non-microsoft-email-apps-c5d65390-9676-4763-b41f-d7986499a90d