The application itself looks absolutely amazing, and the authors should be commended for it. That said, the FAQ mentions that the for-profit model is designed to avoid ads in the client and to "take a strong stance on privacy", which is a little bit disingenuous given that the client exclusively relies on an email service provided by the world's largest advertiser and email data miner. Yes, Mimestream itself isn't us…
Come on, they're explicitly saying that they're charging for the service. It's pretty clear that when they're talking about privacy the caveat is that they won't suck your data into another sink (them) _other than google itself_, being the provider of gmail and all.
I'm all for privacy and informing the user but they're hardly playing word games here.