I may be wrong here, but different email providers sometimes use different emoji sets than the one used to send an email. In those cases, the emoji won’t appear - as you’ve shown above. One of the three web-based email clients I use all use different emoji sets and putting an emoji in a message to myself that will be read by one of the other clients do not appear. 
Even within Discourse you can choose between several emoji sets.
I’ll do some more experimenting to see if I can find a work-around from the 3 providers I use, but I rather doubt if I’ll be successful. One can’t expect every email provider to use every emoji from every set from every emoji provider. On top of that, many emojis with the same name can have slightly different images from set to set.