Emails are not good at conveying moods, personalities and feelings. That is why smileys (or emoticons) were created: to specify one’s state of mind (humour, sarcasm, irony, etc.) and add a little warmth to email messages.
Now, some email users may find smileys fun and useful when it comes to expressing something subjective in a certain number of contexts. However, in general, you should not use smileys in serious exchanges. They can lead to a form of familiarity your correspondent may see as being out of place. Limit smileys to non-professional messages.