Test Mode for SMTP

We've extended the test_mode feature for outbound SMTP to allow you to test your outbound SMTP configuration without actually sending the email.

Previously when an account was created it would automatically be put into test mode. As a site went live you could disable the test mode functionality.

With this change you can now enable test mode at any time for a specific message by adding the x-cloudmta-test-mode: true header to the email.

You can also enable it using the API by setting the test_mode option to true when sending a message.