I have a couple of EC2 machines running in a VPC. Sending a nightly admin status to the Corporate Exchange Server, in the home office - not AWS, works just fine. A new app that I'm developing wants to send a dozen email to various people at a time, and this is when things went strange.
Sometimes 4 emails would go out, sometimes 7, or whatever. Message is always a connection failure. After much debugging it turns out that is an expected behavior with EC2.
See the form at https://aws-portal.amazon.com/gp/aws/html-forms-controller/contactus/ec2-email-limit-rdns-request to ask for permission from Amazon.

Hpefully this will save someone some frustration.
Nice to know, thanks Alex.

João Rosado
You should also use www.sendgrid.com (SMTP Relay service in the cloud).