Background Activity and Push

How come K9 needs to stay as a Background Activity for it to fetch (push) the mails but Outlook doesn’t need to remain as a background activity to push mails?

Keeping on as background activity, K9 consumes a lot of battery and I don’t really understand how other mail clients do the same thing without staying in background.

I’ve disabled polling, and only push is enabled. Is it possible to push the mails without keeping in as a background activity?