> I thought that sending email from EC2 instance was not allowed and only option was using their SES service.

Amazon EC2 throttles traffic on port 25 of all EC2 instances by default, but you can request that this throttle be removed: https://aws.amazon.com/premiumsupport/knowledge-center/ec2-p...

Doubt that AWS would allow anyone to send on behalf of 3 parties like Helm appears to do though.