I'm building a multi-tenant CMS application that will be hosted on Heroku (which uses Amazon EC2, etc). I will also be adding the ability to register/transfer a domain through the application. I would like to add the ability for my customers to be able to have email hosting included and import/manage email addresses through the application.
I have been looking into open source mail servers (such as Postfix) and the feasibility of hosting email. I don't think Amazon EC2 allows hosting your own email server, so I guess I will also be looking for a place to host this email server. What sort of advice can you give me on the topic of hosting an email server for multiple customers/domains?