I'd be curious of stats via the API as well as stats you guys are processing on postmark as well. Not so much to change my life, but an impressive view into the life of a processing system.
Starting with the type of stats you mentions is great... rates at which the API is being called throughout the day overlapped with rates at which that volume is spam.
I could envision a nice looking bar chart (green) for every hour showing the standard volume bell curve ramping up at 6 am and back down around 4pm and then overlayed on that is another set of bars (red) that show how much of that mail was spam, and how much was (orange) ham or something like that.
Would just give a nice pizzaz to the service and by extension postmark I imagine.
I suppose if you throw what I'm saying into a pot and boil it down, I want you guys to become like Akismet. Every WordPress user knows what it is, everyone uses it and when you find yourself in a non-wordpress environment you reach for the Akismet APIs to help you out anyway.
I figure you guys can popularize postmark via a similar route.
because while I can't see what people are testing, given that it's friday afternoon, I'm betting there's a lot of "asdfasdf" going on :)