If you email a prospective (or even a current) employer, make sure you think about your content before hitting send. The New York accounting firm KPMG LLP says that about 1 of every 10 thank you messages (emails) they receive have some sort of smiley face in them :).
That may be OK if you're applying for a job at Apple (or maybe not) but it surely does nothing to accelerate your credibility.
During every step of the hiring process, pretend that your prospective employer is looking for any reason not to hire you. They really are. You cannot let them down at any level because they will find someone else. Don't let a ;) stand in the way of getting your dream job.