If you have a Mailchimp transactional mail (formerly Mandrill) account, you can use this plugin to route all WordPress emails through Mailchimps servers. This can help improve email deliverability and ensure that your emails reach your users' inboxes.
Fill in an API Key and a verified sender and it works, when enabled.
The API Key is stored encrypted in the database for security, so when your database is compromised, your key is not (and people can not instantly start spamming using your account).
For this reason it is important to note that when you change your NONCE_KEY in your wp-config.php file, you will have to re-enter your API key on the settings page.