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There are a few possible reasons for this error message:

Incorrect username or password: Double-check that you are using the correct username and password for your Gmail account. Make sure that there are no typos and that your password is correct.
Security settings: Gmail has strict security settings that may be blocking your application from accessing your account. To fix this, you can enable "Less Secure Apps" in your Gmail settings.
Two-factor authentication: If you have two-factor authentication enabled on your Gmail account, you will need an app password to use with JavaMail. You can generate an app password in your Google account settings.
Blocked account: If Gmail detects unusual activity on your account, it may temporarily block access to your account. Wait a few hours and try again later.
Network issues: Your network connection may be causing issues with your login attempts. Check that your internet connection is stable and try resetting your router.

Overall, the error message "Username and Password not accepted" can indicate a variety of issues, so it's important to double-check all of the possible causes and troubleshoot accordingly.