suggest & integrate 3rd party email sending service (we don't want to run our own mail server)

We run a website on the LAMP stack (linux,apache,mysql but JSP instead of php) We send email for various activity on our website. For eg. new user welcome email, inviting friends email etc. We want to leverage a 3rd party service for reliably sending email and ensuring delivery of the email (without it getting marked as spam by email providers). We need help selecting such a service. Mention the services you recommend and the reason why in your comment/message before you bid. After that we need help setting up and using that service for our current emailing needs. Ease of use, cost etc are all useful criteria for us.