Have you seen Incoming Webhooks?
See Incoming Webhooks that allow you to easily send an email without writing code
Messages sent using this API do not integrate with the rest of the Mixmax platform (i.e. won't have features like tracking, smart variables, the message won't show up in the live feed, etc.). If you'd like to integrate with the rest of the Mixmax platform, you'll need to create a
message resource and send it. Use POST /messages to create a message and then POST to /messages/:id/send to send it.