You’ve probably been tapping the Trash button at the bottom left to delete emails, but that’s a two-handed job: one hand to hold the iPhone, the other hand to tap the Trash button. There’s no way to do it one-handed unless you have massive hands and great dexterity.
But… as usual, there’s another way, and here it is: just push the message to the left, exposing another Trash button!
You can easily push the message to the left with your thumb, expose the Trash button, then give it a tap, all while operating the iPhone with one hand. (Yes, this is a right-handed world. Sorry about that.)
Pushing the message to the left also exposes the Reply/Forward curvy arrow button and the Move folder button, in case you need either of them.
Here’s a movie showing it in action. You should try it.
I don’t know when they added this feature but it’s there in iOS 14.3. Very, very handy.