Language is a means of communication, buts its only as effective as the person using it.  Without inflection and emotion, language loses a lot, making text communication one of the poorest forms.  While emoticons can be informal, they may actually be the best way of bringing additional meaning to your emails. 

Until keyboards can actually perceive the emotional content of our digital messages (not so far off!),  emoticons may be the simplest method of clarifying tone.  I've had to let go of my own perception that emoticons are silly.  They may currently be our best tool for elevating the emotional clarity of digital messages. 

Obviously you don't want to go overboard with emoticons, but a smiley face here and there never hurts.  It's never going to replace a an honest, human smile but it may be your best text-based alternative.