As reported by Neowin, couple of weeks ago, people started noticing that apps such as Outlook, Thunderbird, and other email clients started prompting them for their Google passwords. When they would re-enter their Google password, it would get rejected saying it was incorrect.
Google started locking down its email service and how it connects to third-party email clients, finally retiring “less secure apps”. When enabled, it allowed you to use your main Google email address and password to sign into an email client, weakening the overall security of your Google account. You can still use Google on third-party apps, but the app must support either “OAuth2” or use an app-specific password.