Hello together. Created this Account just for this:
Please add "mtalk.google.com" and its variations (here a Reg-Ex for it: ^((alt)[0-9](-))?mtalk\.google\.com$).
It is used by some Apps like WhatsApp to give the user Push Notification. If it is Blocked the Notification only appear after Manually opening the app. Which is kinda bad for a Messaging App
Here a Post discussing it: Android phones home to mtalk.google.com - #5