Hi Sergo,
thanks for your help.
I tried to do as you told me but I got stuck in annother problem.
I modiefeid the main.cfg with the following:
sender_dependent_relayhost_maps = hash:/etc/postfix/relay_by_sender
This brings me to the follwoing error:
warning: unsupported dictionary type: hash. Is the postfix-hash package installed?
error: unsupported dictionary type: hash
warning: hash:/etc/postfix/relay_by_sender is unavailable. unsupported dictionary type: hash
warning: hash:/etc/postfix/relay_by_sender lookup error for ""<>""
warning: sender_dependent_relayhost_maps lookup failure
Thanks for your help