How can I easily setup a SMTP reverse proxy from EC2 to SES?

You can use Nginx as an SMTP proxy server. Read the link, but it should look something like this

worker_processes auto

mail {
    server_name mail.example.com;
    http_auth   localhost:9000/cgi-bin/nginxauth.cgi;

    proxy_pass_error_message on;

    ssl                 on;
    starttls            on;
    ssl_certificate     /etc/ssl/certs/server.crt;
    ssl_certificate_key /etc/ssl/certs/server.key;
    ssl_protocols       TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers         HIGH:!aNULL:!MD5;
    ssl_session_cache   shared:SSL:10m;
    ssl_session_timeout 10m;

    server {
        listen     25;
        protocol   smtp;
        smtp_auth  login plain cram-md5;
    }

    server {
        listen    110;
        protocol  pop3;
        pop3_auth plain apop cram-md5;
}

     server {
        listen   143;
        protocol imap;
    }
}