Mailgun

Mailgun is a transactional mail service which was launched by Rackspace, but now operates as a standalone business.

The service is entirely free for up to 10,000 system generated emails, and also has an official, free, WordPress plugin in the repository, making it a nice choice for small to medium sized WP sites to offload their system generated mail (comment notifications, signup mails, new password emails and etc) to a third party.

We often use Mailgun for WooCommerce clients, where invoice deliverability is regarded as very important.