I set up a MTA on EC2 with postfix using this guide:
It has been working for me. I've tested it locally on the MTA machine with telnet. I've also tested it by having 3rd parties (using gmail) email it.
I've noticed that when I send an email the first time it is successfully received by the mail server. If I send a duplicated email (same subject + body), the mail server doesn't show any activity in postfix's /var/log/mail.log and it never receives the email. If I send a second email with varying subject + body the mail server it always receives the mail. I've tried commenting out a lot of the restrictions I put into place via the guide in main.cf, but it hasn't helped.
If I test the same thing locally on the mail server box, it receives the duplicated emails successfully.
Any idea as to what's the difference between sending an email locally via telnet vs gmail.com?