Using Sierra OS. I have a gmail account and i connected it to the mac mail app. I can receive emails just fine but I get this error message when I send a message: https://imgur.com/a/D6d0Xr2
I deleted every server on smtp list and added a manual one but it still wont work: https://imgur.com/a/sgmudcQ
Best Answer
Your credentials are not working with gmail. See the suggested gmail help article for troubleshooting. Try logging in with a web browser on your computer first. It may be that 2 factor authentication is causing issues.