Postfix sender_dependent_relayhost_maps: do not relay a specific address
Solution 1:
In Postfix 2.6 or later, I guess you can return the keyword DUNNO, which is documented in sender_dependent_relayhost_maps topic.
[email protected] DUNNO
@mydomain.com ses.amazon.com
If it doesn't work and you use Postfix 2.7 or newer, I suggest you to replace sender_dependent_relayhost_maps with sender_dependent_default_transport_maps. The latter provides more flexibility:
# /etc/postfix/main.cf
sender_dependent_default_transport_maps = hash:/etc/postfix/relay_by_sender
# /etc/postfix/relay_by_sender
[email protected] smtp
@mydomain.com smtp:[ses.amazon.com]