Best of day: Some mail clients display excessive whitespace around message body

[dreikanter]

1. OnePlus 3T running OxygenOS 5.0.2 (Android Oreo 8)
2. Best of day mailing list
3. Received and opened the email using native email client apps
4. Expected: I presume the email layout should save screen estate on smaller mobile devices by hiding gray whitespace around message body. Border line is unnecessary as well since there is physical border of the screen anyway. Gray whitespace suppose to appear on wider screens only.
5. Actual: Gmail app displays ~5px horizontal whitespace. ProtonMail app displays even wider whitespace, both horizontal and vertical.

┆Issue is synchronized with this Trello card