[qnickmans]

disclaimer : I'm involved with www.mailjet.com.

Mailjet is "merging" transactional and marketing email services. Transactional via API and on top of our APIs we build web interfaces for marketers to manage contact list, html and stats pages.

We charge per email send, whatever its "origin".

One need to know that managing marketing emails and transactional emails is not the same in daily operations. Marketing emails are send to "lists" and there is an important effort a reliable provider needs to put in place to avoid spammers come on your platform. Lists need to be double opt-in etc. and a lot of marketeers don't follow these strict rules. The provider (here Mailjet.com or Mailchimp) has to develop and invest in a lot of work to detect 'spammers' before and even during the sending process, interact with its support teams to educate customers, etc. Their job is to guarantee to ISPs only high quality, opt-in messages are send out. That's the only long term battle a provider needs to fight for.