Postmark (http://postmarkapp.com) and Mandrill (http://mandrill.com) are web services that provide peace of mind for all transactional emails that your website sends.
Both services offer officially supported WordPress plugins that you can use to integrate their services with your WordPress powered site.
wpMandrill
Introduction: http://blog.mandrill.com/own-your-wordpress-email-with-mandrill.html
WordPress.org plugin page: http://wordpress.org/plugins/wpmandrill/
Postmark Approved WordPress Plugin
Introduction: http://blog.postmarkapp.com/post/8778033288/wordpress-smtp-problems-are-solved-with-postmark
WordPress.org plugin page: http://wordpress.org/plugins/postmark-approved-wordpress-plugin/