Anyone who has sent a humorous email that has confused — or worse, offended — someone knows the danger of trying to be funny in an email. Email does not convey tone.
How your message sounds to you when you type it has no relation to how the reader will interpret it.
Keep business email straightforward.
Pointing out that something's funny by using an emoticon can appear juvenile.
If you need to share your irresistible sense of humor, save it for phone calls or in-person meetings where tone can be more easily understood.
More info on Harvard Business Publishing