I find myself wishing that full page swipe gestures brought me back to the previous screen rather than navigating between emails.
I like the customizability of the “On Delete” setting — perhaps something similar could be done for swiping back? For example, “On Swipe” could have two options:
If you swipe directly from the leading edge, you’ll go back to the Smartbox/Mailbox. If you swipe in the middle of the screen, it’ll take you to the next/prev message.
Let me know if you’re not seeing that behaviour or if I’ve misunderstood!
The edge swipe behavior works great, yeah. I was thinking specifically about the swipe-anywhere-to-go-back gesture. 
It’s hit or miss these days whether apps support it. Definitely more common on apps needing frequent push/pop interactions (like Twitter, Reddit). Totally understood if it’s an intentional decision to not support it.
Ahh yes I understand what you mean now; Big News has this gesture too.
With Big Mail I’ve been trying to lean into the system controls as much as possible and so have gone with how the stock controls work. That said I’ll take a look and see if it’s something that can be done!