Under global on the MTA tab the authentication box is checked.
Under servers on the MTA tab the authentication box is checkes.
Under domains GAL is set to internal.
I assume this means that auth is enabled.
Restarted postfix via
/opt/zimbra/bin postfix stop
/opt/zimbra/bin postfix start
I get a prompt for a password from the client the first time I try to send, thereafter it just refuses to send.