Basic Setup: Installed and set to Mailchimp, but ignored

I have a fairly fresh site which was working fine with the default mailer.
Includes Drupal Commerce and a custom module as well as Symphony mailer.

I installed Mailchimp_transactional and followed the instructions to route all mail through that . The test mail works just fine, but nothing else seems to go through Mailchimp.

Am I missing a step needed to kick-start this?