Remember, you can add value by combining valuable services. Sell one as a loss leader to bring in the users and the other is your revenue stream.
http://developer.postmarkapp.com/developer-inbound-parse.htm...
Also, if you use Gmail forwarding to foward emails into Postmark, you get the benefits of their spam filtering before it even hits our system.
That said - we watch for spam-like activity across our entire system.
Do they (or a similar provider) already offer a better solution? I didn't find anything in their docs but I might have overlooked it.
We use a combination of algorithms and humans to determine "unusual" activity (not just spam or forged emails) and act accordingly.
We're even proactive about finding customers who break into new send volume thresholds and offer them discounts. We'd much rather have high quality senders in our network for a LONG time than just keep the highest billing rate on autopilot.