Upvote this feature request to get native UnifiedPush support on Proton Mail
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Support UnifiedPush for android notifications
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/47423924-support-unifiedpush-for-android-notifications
I don't have google services on my phone and having every app permanently connected checking for new messages is detrimental to battery life. UnifiedPush is the open alternative to Google Cloud Messaging: https://unifiedpush.org