> 4. Email delivery period. Using AWS Simple Email Service, you're actually sharing an email server IP address with other accounts. They also randomly rotate t

> 4. Email delivery period. Using AWS Simple Email Service, you're actually sharing an email server IP address with other accounts. They also randomly rotate through their mail servers on each retry attempt, so a simple Greylisting implementation based on matching the full IP address can delay your emails for not just one retry interval, but several (until by chance Amazon happens to reuse the IP of the original delivery attempt).