1. Open your Gmail Account
2. Click "Apps" at the upper right (9 dots) > Click Account
3. Click Sign-in & Security > Scroll down Click "Enable Less Secure Apps"
4. If you didn't see "Enable Less Secure Apps", request to your Super Administrator to turn-on "Less Secure Apps" in the Admin Console
Reference for Gmail Message Header link:
https://support.google.com/a/answer/6260879?hl=en (Super Administrator account)