I’m using the Apple/International set of emojis as set in /admin/site_settings/category/posting and it works whether I click on the emoji or type it in manually.
By copying from another website, you are copying the image, not the code to render it.
Manually typing <:raising_hand_man> automatically renders the emoji. 
You may find if you are using a different emoji set, this particular emoji may not be included. I haven’t checked through the list of available sets so it is quite possible it is in every set.
The problem you may be having may be in the KDE email software and/or browser you are using, not in Discourse. Copying an emoji from a website only copies the image, not the rendering code. Typing raising_hand_man (without the ticks) in your email should show the emoji once the email is posted in Discourse. Is it not?