Yes, you can definitely use an external e-mail/SMTP service. You're not limited to using SES.
I'm assuming that you're running a service on EC2 (or some other service) that allows you to write code in your favorite language and connect to an external SMTP service - this is possible using AWS.
[UPDATE] If you are using PHP on an EC2 instance, you can just use regular PHP with the SMTP server of your choice (Gmail, Mandrill, or any other SMTP service you have to access to really), using something like these two SO answers: