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

Your account will continue working. Microsoft is changing the way you login to it, adding some additional protections as part of the process. As some email clients do not support the required protocols/methods to enable such protection, they will stop working after Microsoft implements the changes. If you use an email app that already supports this, you will not be affected, at most you might have to reauthenticate.

If you app is not supported, you might have to look for alternative. Worst case scenario, you can always use the browser to open your account. There are some generic instructions that you can follow here: 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

You should also take this opportunity to examine the security configuration for your account and make sure to add at least one multi-factor authentication method, or a passkey.