If, like pretty much everyone else, you are thinking of launching your web site/service in the "cloud" on Amazon Web Services? One of the things people don't often realize is that they will not be able to successfully send email from an EC2 AMI's. Why? You need a few things these days to be able to successfully send mail from a server. In most cases you need reverse DNS, SPF record, and a static IP address. At AWS you do not have a static IP or a reverse DNS entry. That is a non-starter. You will need to use an external to AWS relay server or service. Following are 3 services you might want to review depending on your needs.

http://www.authsmtp.com
Subscription service by email volume in numbers of emails, tiered pricing.

http://www.easydns.com
Email forwarding by subscription.

http://www.dnsmadeeasy.com
Email forwarding by subscription.

http://www.rackspace.com good location to build your own SMTP server if you are so inclined and install either open source (postfix) or commercial software. That choice will likely come down to your needs for tracking the data your emails generate. It's difficult, but not impossible, to mirror the functionality of a product like Port25's PowerMTA (http://www.powermta.com/) when it comes to tracking and reporting on email sending and activity.

How to choose? It's all about business needs, volume, and what technical skills you have available to you to manage your services. So, just take the time to look at your options.