The Emily Post Institute recommends that people avoid using emoticons in business emails unless they are certain the recipient will see them favorably. Emoticons can typically seem juvenile in formal business emails. For informal emails, especially those between friends, emoticons are fine to use.
An emoticon, short for emotion icon, is a picture used to represent a facial expression in online writing. They are used to express feelings and improve interpretation in the absence of body language. One of the most common emoticons is the smiley face, which is a colon followed be a parenthesis.
In addition to emoticons, it is recommended that writers avoid abbreviations, such as lmk (let me know), in business emails. The use of all caps in emails is seen as the equivalent of shouting, so this should not be used.