I'm getting Authentication required (in reply to MAIL FROM command)) when attempting to relay mail to [mail.sonic.net]:587. If someone has a configuration for postfix that actually works, I'd love to look at it. Of course, you should eliminate private information.
My sasl_passwd file contains:
mail.sonic.net:587 <my-sonic-account-name>:<my password which happens to include spaces>
One search result claimed that sasl_passwd is 'space delimited' so that might be a problem. No idea how to escape spaces though - you could try the postfix-users list but be prepared for some noob-shaming as I have suffered.
Are you the same person whom I sent my configuration to the other day? If not, then mine might be helpful.