@mydomain.com         ses.amazon.com

@mydomain.com           ses.amazon.com
no-relay@mydomain.com   null?

no-relay@mydomain.com   DUNNO
@mydomain.com           ses.amazon.com

# /etc/postfix/main.cf
sender_dependent_default_transport_maps = hash:/etc/postfix/relay_by_sender

# /etc/postfix/relay_by_sender
no-relay@mydomain.com   smtp
@mydomain.com           smtp:[ses.amazon.com]