When you enable Two-Factor Authentication (2FA), mail clients that do not natively support 2FA, such as Microsoft Outlook and Apple Mail, may be unable to connect to your account.
This is a security feature to ensure unauthorized devices or apps don't gain access to your emails. To restore access in such mail clients, you'll need to generate and use App Passwords.
For more information on App Passwords, you can use the following article: What Are App Passwords?