I am using my Gmail account and smtp.gmail.com inside my web application to test and send email. when two factor authentication is ON for my Gmail account, it fails to send the email, however when I turn off two factor authentication for my Gmail account, the web application sends email successfully. Appreciate any kind of advice.
Create a custom app in you Gmail security settings.
1. Log-in into Gmail with your account
2. Navigate to https://security.google.com/settings/security/apppasswords
3. In 'select app' choose 'custom', give it an arbitrary name and press generate
4. It will give you 16 chars token.
Use the token as password in combination with your full Gmail account and two factor authentication will not be required.
Note: The link in step 2 will work only if you have 2-factor-authentication enabled.
来源：https://stackoverflow.com/questions/26736062/sending-email-fails-when-two-factor-authentication-is-on-for-gmail