How many emails can I send from my Amazon EC2 server?
As far as I can tell the limit is near 0. Amazon's intent is to have all legitimate email senders ask for approval before sending email. If you have an Amazon EC2 account yo can request permission to send email using this form :
Request to Remove Email Sending Limitations
You'll need a static IP address. I don't see a way yet to create a custom reverse DNS PTR record. I imagine I'm about to find many of the EC2 IP addresses are blacklisted. If you have access to an SMTP relay service you might want to consider usnig that for relaying out your emails. The amazon web hosting is likely fine for sending transactional emails but not mailing lists.
If you are running a news letter service try using a newsletter sending service such as Constant Contact