...or you could just sell the software? That's how we used to do it in the Olden Days.

Considering the amount of the personal data still going through email, this is one of the few areas that no one should be giving up to any third party services, regardless of the benefits it may provide.

Encryption with you owning the key is a requirement if it is going to be stored on a third party service. You can provide a subscription model for the server costs if need be.

They actually mention in the FAQ [1] that once they officially release their apps there will be a free and a paid version.

[1]: https://canarymail.io/faq.html

Yes, they do. It's why I don't use them.

Source: https://sparkmailapp.com/privacy

> Accounts are added to Spark through OAuth where possible. Where OAuth is not supported we keep your account username and password on our secure servers. We then use the authorization provided to download your emails to our virtual servers and push to your device. We use Amazon Web Services (AWS) infrastructure to store your data. Apart from the AWS' security policies we take a number of measures to ensure that your data is never read by anyone else. We ensure that all transmission is secured with HTTPS so that no one else can access your data. Your account credentials are stored on secure cloud-based servers using asymmetric encryption.