Other email services such as Gmail and Outlook have this, and it seems pretty simple to implement. Currently, when looking at the inbox on desktop and in the mobile app, you can see the sender and subject of each email. I think we should also be able to see the first few words of the email itself.

On desktop, the remaining space to the right of the subject line would be filled with the actual message body so that the email's content can be ascertained at a glance.

On mobile, the email notification should be expandable to make some the message body viewable. This would allow users to skim the beginning of the email without opening the app and consequently dismissing the notification.

I've personally found this feature extremely useful in Gmail and Outlook, and the lack of it in ProtonMail is noticeable every time I use it.