Symptoms
Outbound TCP port 25 is on your EC2 instance, and Amazon will not open the port for your instance.
Description
Amazon Web Services blocks outbound TCP port 25 by default. While Amazon may lift this restriction, they can refuse to unblock it.
Workaround
If Amazon does not allow port 25 to be opened on your EC2 instance, you must use Amazon SES (Simple Email Service) or a third-party smarthost on an alternative port.