Kakao Customer Service Main
Please refer to the steps below and try again.
1. Check if Gmail POP is enabled.
2. Use App Password: Log in with App Password if you use 2-step verification.
(You can no longer register external mail by only using your username and password since Google login policy will be changed from 2022/05/30.)
3. If none of the tips above solves your problem, please visit the "Allow Access to My Google Account" page and follow the instructions.
For more details, please refer to Google Help.