Resolving AWS Port 25 Throttling
SMTP can operate over ports 25, 587, or 2525 but the default port used in an EC2 instance is port 25. Amazon Web Services’ EC2 instances are throttled on port 25 by default as a spam prevention measure. This can cause connection issues when attempting to use SMTP to relay emails through Postmark in your EC2 instance. There are a couple ways to resolve this issue so that you do not receive connection errors when using Postmark in your EC2 instance.
Have AWS Remove Port 25 Throttling
Change SMTP Port(s) in Your EC2 Instance
Ensure Ports 2525 and 587 are Open for Your EC2 Instance
Finally, ssh into your console and change the ports used for SMTP in sendmail to 587 and/or 2525.