What are transactional e-mails, you might ask? The answer is: e-mails generated by a web application after an user action such as registration, requesting a password reminder, receiving a reply to a comment, etc. The problem me and many developers faced is that sometimes the client's servers are blacklisted and these e-mails land in the users' spam folder. This is where Postmark comes to the rescue, hopefully.
The Postmark API accepts JSON encoded content via a POST request that includes the to, from, message (plain text or HTML) and an API key. The Postmark service is currently in closed beta but you can still grab the library from either this direct download or on its github repository.