Symbols have been used in email subject lines for many years, way before emoji became ubiquitous. And it's not surprising that they are - research shows symbols like a snowman, sun, or star can improve your open rate. These symbols, found in fonts like Wingdings, differ from emoticons, which are typographical displays of facial expressions, such as :-).
A step up from emoticons and symbols, emoji are those colorful mini graphics ranging from yellow happy faces to rocket ships, and they're the latest way to emphasize your point in subject lines. However, they may not be appropriate for your campaign (or display the way you intended them to), so use A/B testing to find out how they affect your open rates.
Learn more about using symbols and emoji in your email marketing in this infographic.