Good working relationships can be irreparably harmed by pushing the send button too soon. Thou shalt not gossip. Your juicy news may go directly from your screen to lots of others' printers. Unlike spoken gossip, it's far easier to trace back. What's funny in person may not be funny in print.
Office humor can sometimes backfire and that's especially true when it's in writing.
It may also create liability for you or your company. Keep personal E-mail traffic to a minimum. Organizations are becoming rightfully more sensitive about how much time is spent on personal E-mail. Treat personal E-mail the same way you would treat personal phone calls. Don't forward messages indiscriminately.