There are various methods of arranging push notifications for email. None of them require the messages to be stored on third party servers.
Any email “client” service that stores your messages and credentials and is not the actual provider should be avoided at all costs.
A real email client connects directly to your provider’s servers and shows you what messages are there. Whenever it needs to do something with your messages (send, receive, notification etc.) it arranges it with your provider’s servers, never with 3rd party servers. If it stores your credentials, it stores them on your device, either in your browser, or in your app data (if the client is an app), never on 3rd party servers.
When you use Spark or others like them it’s like paying someone to check the mail box on your lawn. It’s your own mail box (the provider, for email) but instead of checking it yourself you give a complete stranger the box key and allow them to rummage through your mail and packages before they bring 'em in.
You don’t need this service. Nobody needs this service. They’re taking advantage of the fact people can’t make the difference between a client that runs at the provider, on your device, or on a 3rd party server.