We are happy Postmark customers also, sending a couple thousand transactional emails per day. Postmark delivery rates are of the highest rates I've seen. I also appreciate how easy is to diagnose any delivery problems via their API or the web interface. In the rare occasions we've needed further details their support staff were immediately helpful.

It is the most expensive service if you don't buy in bulk, but is well worth if you want to stop worrying on blacklists and other delivery problems.