Stop local emails being sent to relay host with postfix

Postfix relayhost doesn't send local mail to the relayhost normally:

From postfix.org:

relayhost (default: empty)

The next-hop destination of non-local mail; overrides non-local domains in recipient addresses. This information is overruled with relay_transport, sender_dependent_default_transport_maps, default_transport, sender_dependent_relayhost_maps and with the transport(5) table.

So if it is happenning, it is because your local mail is not identified as local. You need to include your local domain in mydestination = config line. I see you have the ec2-....amazonaws.com there but probably you have defined a local domain different than this one for your LAN. If you don't have defined a local domain, I think (needs testing) if you only add 127.0.0.1 localhost.localdomain localhost in your etc/hosts file and add localhost.localdomain to mydestination= it should be sufficient.