It’s important to know your limits. In this case study we find a situation stemming from SMTP being throttled. This is part of the packet capture I received: The top lines show the previous conversation ending. SMTP successfully sent 3 messages. After the 3rd message the mail server stopped responding and retransmits began. This pattern was repeatable. More than that it was repeatable from other EC2 instances. The only thing between the EC2 instances and the mail server was a router and a firewall.