Got a couple of emails this morning with a bunch of icons or Emoji in the subject area. So I checked on-line and this is how it's done.
Go to https://twitter.github.io/twemoji/preview.html
Click on an emoji that you want to display. In the box that opens you can press Ctrl + C to copy and Enter to close the box or you can right click inside the box and click on copy.
Then for your email, I'm using the Seamonkey email client. Create a new email I click on the word Compose to start a new email and in the subject line wherever you want to place the emoji right click and Paste. I sent a test email to myself. I've used the maple leaf in my example.
The end result:
May not work with all email programs or services also the emoji may look different in some cases. Don't over do it. :-)