I will be deploying on EC2 myself imminently and I bookmarked some multiple notes from another thread that seems to indicate the whole package includes 1) using admin_user: ubuntu, 2) [adding PEM to keychain] (https://blogs.aws.amazon.com/security/post/Tx3N8GFK85UN1G6/Securely-connect-to-Linux-instances-running-in-a-private-Amazon-VPC) (if on a Mac), 3) Adding EC2 hostname to /etc/hosts/ for SSMTP to work, and 4) switching from AWS pem key to your rsa key.
@jolaurin, I am really curious what your findings may be when you get it working.