Adicione a linha "permit_mynetworks" às restrições para permitir o envio de mensagens.
smtpd_recipient_restrictions =
check_recipient_access hash:/etc/postfix/whitelist_recipient
permit mynetworks
reject_unauth_destination
reject_invalid_hostname
reject_non_fqdn_sender
reject_non_fqdn_hostname
reject_unknown_sender_domain
permit