EC2 users face a weird problem some times. They observe random fails in email sending via smtp when they try to send more than 3-4 emails at a time. You can also say it in another word – Email works quite randomly.
This is because of a policy of AWS. On EC2, outgoing traffic over port 25 is throttled by default.
For the resolution of this, you will have to request aws to remove this limit. You can request it here –
https://aws.amazon.com/forms/ec2-email-limit-rdns-request?catalog=true&isauthcode=true