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.
Using Gmail or another account to sign into a third party app may seem secure but it unfortunately slightly weakens the security of the account as you are giving away a password to another platform which may not share the same level of protection. It is always best to use a new account when signing in, even though it may seem less convenient, and you should never reuse passwords. Apple even offers to hide your email and offers a suggested password for the majority of account logins and then stores this information in the in-built password manager. Multi factor authentication is another way of increasing protection and should be enabled with every account that offers it.