Setting up Postfix can be a pain. Here’s the ultimate cheat sheet to setting up Postfix.
For sending you can either use smtp relay with a service like gmail or google apps.
Or you can use dig -x xx.xx.xx.xx to find your PTR record. Whatever domain you’re hosting on your Ec2 instance use this PTR record result for myhostname.
myhostname = ec2-xx…..compute-1.amazonaws.com