It has been repeatedly said that EC2 instances are bad for email service. The reason given is that Amazon's IP ranges have been abused by spammers in the past, spammers who just rented a EC2 instance for a few hours to send spam. Thus Amazon IP's should now carry a negative reputation in IP address based DNSBL's.
I can't say if the above about Amazon IP's is entirely true or not. What you can do is verify your IP address with a tool like MXToolbox's checker.
If you decide to set up a mailserver on EC2, then hunt around on this site for best practices. My short list of this you absolutely must do (there can be more) is:
- Set up reverse DNS for your SMTP server.
- Set up SPF records for your domains.
- Go over your SMTP configuration with great care. Unfortunately there is quite a lot to look for. Off the top of my mind the most important are: a) that your HELO header is correct. b) that your SMTP server will send email for authenticated users only.
For a more comprehensive discussion of best practices, see this large Stack Overflow thread.
Reverse DNS is usually handled by the netblock owner, i.e. Amazon. See this from Amazon regarding reverse DNS.
Much of the above will have to be re-done if your IP address changes. So be sure to get a permanent IP in Amazon's cloud. All in all, I don't think cloud hosting is ideal for SMTP service...