postfix via stunnel: 553 Relaying disallowed

postfix via stunnel: 553 Relaying disallowed

I followed this link (  http://www.code2control.com/2013/12/setup-postfix-zoho-mail-ubuntu ) and setup postfix to relay though zoho mail server successfully. It had one catch in that I had to specify an additional header, "From"  for every "mail" command.

To  overcome the restriction, I added the following in /etc/postfix/main.cf 
  1.  sender_canonical_maps = hash:/etc/postfix/sender_canonical 
and sender_canocial with the following entry.
  1. localuser@localdomain          zohouser@zohodomain

I could see the address getting rewritten as zohouser@zohodomain in /var/log/mail.log but I still see
  1. 553 Relaying disallowed (in reply to end of DATA command))




Any ideas? Thanks.