I added an email address and forward to my Gmail in Opalstack.
In Gmail, go to settIngs > Account and Import > Send mail as > Add another email address > Name: Michael > Email address: email@example.com > Clicked (Next Step) > SMTP server: smtp.us.opalstack.com > Port: 587 > Username: NAME > Password: ******* > Add Account > I get an error:
Authentication failed. Please check your username/password.
Server returned error: "535 5.7.8 Error: authentication failed: authentication failure , code: 535"