Here is the form to request a custom PTR record for an Amazon EC2 Elastic IP. It's included under Request to Remove Email Sending Limitations, which is not helpful.
This is good when setting up a mail server in Amazon's cloud. Without this request, the reverse DNS lookup will the default PTR record - something like ec2-50-16-219-8.compute-1.amazonaws.com. When the reverse DNS record doesn't match the origin domain, mail providers like GMail are likely to mark your mail as spam.