Conditional sender rewrite based on recipient address

[nicolas]

Hi,

I'm running Postfix 2.9.6 and I'm trying to do a conditional sender 
rewrite based on recipient address. Machine sends e-mails as 
r...@internal-domain.com by default, but if the recipient is 
u...@external-domain.com I want to rewrite sender to be 
r...@different-domain.com.

To do so, I've tried doing the following:

1) In master.cnf I added:
   toext     unix  -       -       n       -       -       smtp -o 
canonical_maps=hash:/etc/postfix/generic_toext

2) In the transport_maps map I added (and postmapped):
   u...@external-domain.com   toext:

3) Content of /etc/postfix/generic_toext (postmapped):
   r...@internal-domain.com    r...@different-domain.com

This seems not to work, as the original sender is still the same. As per 
[1] something like this could be done by using the canonical_maps 
parameter, however it doesn't seem to work in the way I'm using it. 
Could someone point me what's wrong in the configuration above?

Thanks.

Nicolás

  [1]: http://www.postfix.org/ADDRESS_REWRITING_README.html#canonical