Why using a 3rd party API when you can set up your own emailing service for a fraction of the price?
For your notice - you are not allowed to send explicit images on any newsletter service. Explicit images in bulk emails is a big no-no. Also, using explicit keywords could get emails sent to spam. So you always try to avoid those two. By the way, that includes using your own service.
Feel free to contact me on email: webmaster [at] seobackyard {dot} com if you need any consultation to set up and manage your own newsletter.