When I did the manual pipe it was clear that IMAP and Mailparse weren't enabled after all on the server. Looks to be working now so thank you for the help.
Looks like you created a request here https://requests.blesta.com/topic/support-for-hosted-transactional-email-postmark-mandrill-mailgun-etc
We are considering transactional email integration, it's something that I think we are likely to do, it's just not a very high priority at the moment. We'll keep an eye on your request
Check also that PHP IMAP and Mailparse are in your CLI PHP. Your ../logs_blesta/ error logs may have more information. Trying to pipe manually via SSH as in the test case described in the docs should help to narrow it down.