There’s an Ask Metafilter question about how to write friendlier emails and the answers are quite good. It’s hard sometimes to convey sarcasm or humor via e-mail because the other person doesn’t have the benefit of your tone of voice or facial expression. This leads to misinterpretation.
Here’s some great advice for writing business e-mails
“Humor and sarcasm rarely translate in e-mails. The solution is to be succinct: Limit yourself to saying what’s necessary, only. Just like anything else, the potential for malfunction or misunderstanding increases with every added layer of complexity â€” in other words, the more you write, the more likely you’ll write something that can be misread. Keep it short and simple.”