I've only tried Postmark ( http://postmarkapp.com/ ), but they've always done exactly what I needed with no problems so I never bothered looking elsewhere. Their Python library is great (they even have a Django email backend for it).

If they don't have a wrapper library for your particular language, their web service is simple enough that you can write your own in less than a hundred lines: https://github.com/sjl/clojure-postmark/blob/master/src/post...

(I'm not affiliated with Postmark, I'm just a happy customer)