Talking about run rates in the first 6 months is iffy. The run rate difference between #5 and #26 (where they are now on the Mac App Store) grossing are huge. Order of magnitude huge. How much of that $500k was when they were featured and making their initial splash? What's their daily run rate
now at #26? What's the trend? I'd wager it's been down steadily if it's anything like the iOS app store.
I don't know about the Mac App store, but on the iPhone, their daily revenue was in a state of decline. There is SO much gravity in the app store. Stuff falls-- it's the natural state for almost all apps unless you have an ARPU or some marketing hack to counteract it.
I'm not saying they were in a crappy position-- though I'm not sure that a $2.99 / $9.99 email client is a good business bet for 5 engineers. Email is complex/weird and users will want support-- with a LTV of $2/$6, it's really hard to afford to support them well. You can't afford marketing. You're competing with $200k comp packages for your team.