I started using Spark yesterday, and was really hoping that it would become my favorite iOS email client. Unfortunately, I am unable to connect to my company's Exchange server. When I attempt to add the account and enter my credentials, I get a message telling me to use the "Back" button and select Google from the Connect to Service screen. I don't know the nature of the problem, but it might help if Spark provided the ability to enter specific domain and server settings, as other email apps do. I contacted Readdle tech support, who asked me to turn on "verbose logging" and send them the file it produced. They also asked for my Exchange login credentials so they could test the issue, but I'm not willing to share that information.
I also had difficulty adding my Outlook email account. If I selected "Outlook" from the Connect to Service screen, the account creation failed. If I selected "Exchange," I was able to add the account.
I realize the app is new, and I have no doubt that these issues will be resolved in future updates. Spark has some great features, in my opinion, but it's not practical for me in its current state.