"You’re trying to build (or maintain) a relationship with the people you email. Just as you would normally not end a spoken conversation without saying goodbye, you shouldn’t do it with email (barring a few exceptions). Granted, the sign-off is just a small part of your message. What’s most important is having something to say that is interesting, sincere, and not focused solely on you."
"That being said, the sign-off is the last thing the recipient reads—so it can be the “cherry on top,” so to speak. Done right, it’s like the motivating conclusion at the end of a really great presentation."