Some people feel that emoticons (also called smileys) are unprofessional, childish, silly. BUT if you are writing an informal email in a second language they can be a useful tool. Sometimes a jokey comment or a short sentence can be misinterpreted because the reader cannot see your facial expression or hear your tone of voice. In that case a smiley can help the reader know you are not angry or offended. I use them a lot when writing to students I know well because I am quite direct and undiplomatic.
If you are writing a formal email to someone you do not know, the rule of thumb would be that if you feel you need a smiley to make sure you are not offending someone, you could always rewrite your sentence to be more diplomatic, friendly, neutral. As always avoid cultural references, idioms and jokes unless you are totally sure the recipient will understand and appreciate them.
And the best tip ever would be to never write emails when you are angry. I guess each one of us has learned that one the hard way. It has nothing to do with smileys but I felt I would pop it in anyway!