Which SMTP relay to replace Mandrill
This conversation has been taking place on the CiviCRM Partners' list in the past week.  Fortunately, some folks from the list created a wiki page to track the best options, which is available here: Mailing Providers.

I like Amazon SES, which termdark referenced (I'm the author of that documentation they linked).  However, CiviCRM partners with much larger installations don't like it because it lacks some high-end features (e.g. sub-accounting) and because they can vary their throttling.

If you're only sending 600 messages/month, you can probably use your web host's SMTP servers.  If you're on a VPS, you can set up Posfix yourself - you're unlikely to get blacklisted with such a low volume if you configure it right, and being able to configure Postfix is an important part of how to set up Amazon SES.

EDIT: There's a lot of interest amongst the partners in SparkPost.  They offer 100,000 free messages a month, and support many of the high-end features.  There's also an extension under development by the folks at CiviDesk to use the more efficient (and easier to set up) API for bounce handling.