It is either a problem with Mail or with Print. Quitting Mail app then relaunching it may help.
Can you print other documents properly?
First, close Mail and re-launch it. Try printing now.
Second, try rebooting your Mac.
Try taking a screen shot of the email and printing that. Use the Key combo Cmd Shift 4 then Spacebar, or the Screenshot app in Applications-Utilities-Screenshot.app for taking the picture of the email.
You can also try Disk Utility First Aid on your boot volume to see if that will correct the problem.
As a last resort you can boot into Recovery and reinstall the OS. See MacOS help for instructions on Recovery. No data will be lost.