* Sends _statistical data_ to several services known for bad privacy policies (Google, Facebook), also there's no way to opt out.
* Automatically creates an acount with the first address entered and subscribes you to their newsletter.
* Stores credentials for your email accounts on their servers.
* Stores your emails on their servers to push them to your devices.
* Server infrastructure seems to be located in the US.
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Just the first few points "General Statistical Information" and "Personal Information" make me shake my head. Spark connects to several services like Google Analytics, Facebook and Amplitude and send _statistical data_:
> We use third party services, such as Google Analytics, Facebook Analytics and Amplitude, to collect and analyze how you use Spark.
> The first email you add to Spark is used as your username. We might use that email address to reach out to you periodically with information about features, updates, announcements or to request your feedback.
So, whatever address you enter, you subscribe to a newsletter without even knowing it. As stated in the policy you can only unsubscribe by contacting the support which leads to another problem:
> We may use this email to contact you later with updates about your request, with information about related features and other announcements.
So, another newsletter then?
> We then use the authorization provided to download your emails to our virtual servers and push to your device.
Huge downside, even if the policy states that the emails are stored encrypted. Also, even if some information is stored encrypted, the server infrastructure seems to be located in the US. I think I don't have to tell you what this means...
Edit: this is the most important thing. They say they encrypt the credentials but they use them to retrieve your emails from the servers. This is kind of an opposition until Readdle discloses how this is working without breaking the encryption.
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Conclusion: Use another email client!