You have the option of changing your account settings, and one of the preferences is to use encryption, for both incoming mail and outgoing mail.
When you are in the e-mail app, pull down the menu in the upper left corner of the screen, and choose "Preferences & accounts".
Tap the account you want to modify.
On the screen that appears next, scroll all the way to the bottom, and tap the button labelled, "Change login settings".
On this screen, you have the option for SSL or TLS encryption of your login information for both incoming mail server and outgoing mail server.
Your mail server from whatever service or ISP you are using has to support login encryption - I think most do these days, but it may not work. Just turn it on, and try to get and send some mail. Worst case is you get an error message.
I don't think this encrypts all your e-mail data going to and from your device - just the login "conversation" that happens when you get your mail.