Earlier quoted context omitted.
I dare you to look at the API for some of the "traditional" email service providers. I've had the displeasure of working with a few from some of the big names and they are real DailyWTF material.
I have worked with Constant Contact and it was a Constant Headache. My favorite was when they took the API down for maintenance and maintained HTTP 200 OK status codes for requests but instead of XML returned it was an HTML page saying they were down. I took a screenshot of the page because it was so surprising: http://www.jongales.com/blog/2009/10/17/constant-contacts-in...
Silverpop is similar.