Because there is no "tone of voice" in an email, sarcasm can easily be misinterpreted by the receiver.
"John, make sure your office is clean when clients come in - you might scare them away with all that mess."Bad example: This is bad because it may seem like John is being reprimanded, even though the sender may just be giving him a "heads up" for next time. "John, make sure your office is clean when clients come in - you might scare them away with all that mess :)"Good example: When in doubt, use a smiley face at the end of the comment to soften it up a bit.