When considering the use of emojis (or emoticons) in Subject Lines or email content, the main concern is how reliably they will display across popular email clients.
If a character you use isn't supported in the email client of your recipient, the recipient will see a ☐ character instead. Even if an email client is able to render the small image (i.e. the emoji) you've added to your email, the image may not render as desired. For this reason, Klaviyo recommends testing out any new emojis you'd like to use by sending test campaigns to a preview list or using a tool like Litmus.
How Emojis Render Across Different Email Clients
Here is an example of how the [✌] emoji will look in the subject line of an email viewed in Outlook 2013 and Gmail:
Here is a preview of how a myriad of different popular emojis render across different desktop, web, and mobile clients:
How to Use Emojis in Your Emails
Klaviyo supports all emojis! A great resource for copy and pasting emojis into your templates is Copy and Paste Emoji.