We run a wiki in an EC2 that sits behind an Elastic Load Balancer, the EC2 does not have a public IP and traffic from internet is routed to the ELB through Route53. We noticed that the wiki was not sending emails and while troubleshooting I realized that if I add an Elastic IP then the wiki does send emails just fine. It's maybe worth noting that the database and EC2 are part of the same VPC, region and account, but the EC2 and the Simple Email Service belong to different AWS accounts.
I checked the security group of the EC2, the ELB security group and the VPC network ACL, they all allow outbound traffic everywhere, which leaves me at a loss as to why would it not be able to connect to SES
Ideally we wouldn't need a public IP on the EC2 in order to send email.
Is there anything I should check or is there a known way to make this work?