I used the smarthost listener also some time ago. Maybe you have the same problem like I had.
Take a look at the relay password file. Default path is "/etc/postfix/relay_passwd", as far as you haven't changed it in the listener file ($saslPasswdMapsPath).
You might have the same account with 2 different passwords in it. If that's the case, just remove the file and rerun the i-MSCP setup.