in /etc/postfix/main.cf:
sender_dependent_relayhost_maps = hash:/etc/postfix/relayhostsin /etc/postfix/relayhosts:
example.com [smtp.provider.com]:
domain2.com [smtp.live.co.uk]:Once you have relayhosts file as you want it run the following command
postmap /etc/postfix/relayhosts && postfix reload