First of all, we suggest you start by reading the following article 4 questions answered about emojis in email subject lines.

Then, it is important to keep in mind that:
• Emojis are not all supported; they are more and more supported, but not 100%.
• Not all symbols are supported, so you need to test thoroughtly.
• When the symbol cannot be displayed, a square will take its place (☐).
As for message reception, or spam, there is no way we can be certain that the message was received or not, and nothing garantees the delivery of messages. In addition, for delivery, each domain is configured differently, and so are inboxes and anti-spam filters. So each company and client is probably subject to different rules that either accept or decline special characters such as emojis.

So to reply to the question ... "Yes you can use them, BUT test!"

Other links of interest

So our best piece of advice is "TEST, TEST, and TEST again!"

However, there is a slightly more advanced method that could allow you to use any Emoji by following these steps : 

1. In the additional options of your message, you must go to the "Parameters" section and add one by giving it the code and label you want. (for example: Myemoji)

2. Copy the emoji you want to display in the default value, the preview value and the value for the tests. You should get results like this:

3. In the object or place where you want to display the emoji, you must insert this piece of customization code: [[=Message.Parameters.ParameterCode;]]. Obviously, it is necessary to replace "ParameterCode" by the code that you gave to your parameter. In the example here, the code is Myemoji and the customization should be [[=Message.Parameters.Myemoji;]].

4. Save and test your message which should now display your emoji.