I had the exact same problem with the 503 error and the AUTH not available error. If Sander's hint doesn't work, edit the site.ini file that you also quote in your post.
Simply leave TransportUser and TransportPassword empty and see if e-mail notification works again.
Regards Robin
Board member, eZ Publish Community Project Board - Member of the share.ez.no team - Key values: Openness and Innovation.