Today I Learned

A Hashrocket project

EC2 throttles outbound port 25 connections

Give this a shot on your ec2 instance:

while true; do echo 'try!' &&  nc -w0 $SMTP_HOST 25; done

I get

try!
try!

And then an infinite hang.

If you are working with SES like I was, a workaround is to connect to port 587 instead.