I hope you’re aware that Spark is not a normal email client. They store your emails and your email login credentials on their servers, snoop on your emails and share their findings with Google Analytics, Facebook and Amplitude. I hope you ditch them ASAP.
Not sure what kind of app you’re asking for, webmail app, desktop app or mobile app?
Just to clarify, email clients don’t work the way Spark does, you don’t install it on a server and connect to it with an app. An email client is a standalone app that connects directly to your email provider with nothing else in-between.
An email client can be a web app, which is hosted on a webserver, but then you can and should use the one already offered by your email provider, which is Fastmail, not a third-party like Spark.
So what’s the difference between the webmail offered by Fastmail, Spark, and an independent web app like Round Cube?
A web app like RoundCube is a tool that can be used to get and send your email. Being a web app it is served from a server, but the app runs in your browser. Your emails and login are not stored on the server that holds the web app, that server’s job is just to hold the app. Once you load the web app on your device (PC or phone) it must only talk to your email provider. Emails must be stored either on your device or on the provider server, never on the server that holds the web app.
Spark breaks that rule. They offer a web app as well as mobile app etc. but emails she logins good through their server and then to your provider. They’re essentially an intermediary who controls and snoops on your email.
Fastmail is your provider, they already have your email and your credentials. They also provide a web app as a convenience. In this case the web app comes from the same place that holds your emails, but only because this is your provider. Nobody else should get between you and your email.