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We use MailChimp at NewsCred and love it. Awesome service, easy to use website and they definately do have an API.

In fact, they have an API for transactional e-mails (i.e. ad-hoc ones to welcome a user for example). So you're not just restricted to using them for bulk emails.

I'm also using mailchimp, I do ~500 emails per month. Not a very high volume, so it costs 3c per email, but the data they provide on the campaigns is worthwhile and especially helpful when I have to report back to my clients.

I use mailchimp since wayback and I like them. The later versions are really robust, but i don't think they have an api. Sounds like you want less of a SaaS?

Having just written an integration into it, I can assure you MailChimp has an excellent API: http://www.mailchimp.com/api/

It's far superior to Constant Contact's API, which consists of two methods: subscribe and unsubscribe (last time I checked anyway).

The mailchimp API is topnotch. The only issue is their admin panel is too AJAX-y and bogs down when your mailing lists are several hundred thousand people.