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I am sure the Gmail password is correct.
Why do I always get the error message "Mail server authentication failed" when testing the Email server using Gmail?
You can try these troubleshooting steps.
1. Login to your Gmail and check if "Less secure app access" is turned on.
2. Visit the website and follow the steps on the page.
You will then get "Test email has been sent successfully." in the result.
Still have trouble with your device? Contact Zyxel technology support team directly!Contact Zyxel Support