Components
Proton mail is a free and open source programme which involves encrypted mail service which allows users to send media and mails from end to end. It is highly secured and assures user of no third party involvement in the encrypted messages which means that even the programmers and operators can't intrude into the privacy of the user. It also makes use of modern day mail service which implies that it will be easy to use.
Proposal
My proposal is for an appearance of a shortcut to the open source programme. This shortcut will appear on two basis:
When the user is not currently on using the app, ie the user is operating on another app, and
When there's a notification from the proton mail app or when the user has received a mail.
This shortcut will appear at the corner of the screen so that it won't interrupt with what the user was doing currently. The size of the shortcut will have to be small but noticeable. So how this will work is that when the user receives any sort of notification either mail or alert, the shortcut pops up as a small icon on a small corner of the screen and the user clicks on the shortcut, then, the message or notifification will now appear fully but will stop or close the other app in which the user was initially using. When the user is done, he/she can go back to whatever they were doing with the phone either by pressing the back button on the phone or by pressing the cancel sign on the shortcut.
Mockups / Examples
This how the screen will be currently while the user maybe operating on facebook or whatsapp or the phone home screen respectively
How the shortcut should look like if added... It could take any structure but let it be small and noticeable
Now when the user clicks on it, it will appear in this form and when the user wants to exit they could click on the cancel sign on the corner of the shortcut. 
Benefits
It will enable the user to use the app at any point in time even when using another android software.
It will kept the user updated of new mails and alerts even though the user hasn't opened the open.
The user would be able to to use the proton mail app simultaneously with any other app he/she was currently using at that time.
The user wouldn't have to stress themselves by opening the proton mail app and waiting for a mail, when he/she could have used that time to be doing something else with the phone.
Posted on Utopian.io - Rewarding Open Source Contributors