There is a security issue with Gmail itself. Go to Gmail, click on your profile icon and “My Account”. In the center of the screen, click on “Your personal info”. Near the bottom, in light gray color, click on “Check your security settings”. Scroll down to “Allow less secure apps” and change the setting to “ON”.
Seeing messages being send without something in the Outbox? This could be a stuck read receipt.
Use this web site: