You can select/copy/paste it into another app that can print. Depending on the app, you may lose some formatting.
It's not something I need to do often, but if printing and preserving the format (as much as possible) is a concern I'd probably use PDF Converter by Readdle. After you select and copy the part of the email you want to print you open PDF Convert and choose the Clipboard options. It shows you how it will look (usually pretty close to the original in my experience) and asks if you want to Convert it. Once converted (to a PDF, don't you know) you have the options to Print it (via AirPrint), email it, save it to another app via Open In, or copy it to iCloud Drive.
{Note: The Convert button in the second image is approximate. I didn't' feel like the extra work it would take to combine two different screenshots, so I just pasted some text where the button appears in the previous screen. It won't look exactly like that.}
I'm sure there are other ways, but I don't know of a better one off the top of my head.