Yes, the username is the one displayed in the the Teammates page, the password is the key I generated (telnet works using this key), and other two fields are exactly the same.
May I know what version of discourse are you using, I’m using beta, I suspect that could be the issue.
Anyways, I’m planning to switching to mailgun, I tried it today, and it works seamlessly (though it’s expensive, but have no other options atm)