Electronic messages are the last place for any kind of uncomplimentary remark. Just don't do it. If you insist on posting a message or sending mail, make sure you do it the right way, use the correct mail class, and avoid spamming people. Read your company's mail policy. If you don't know, ask.
Electronic mail is insecure.
Your most private message can be read by others as it travels through the internet. System administrators at your company, at your intended recipient's company, and at points in between, can read your message. And a security breakin, or a software bug, might allow others to read your message also. Unless you are sure that you are using a secure channel, don't send any kind of information in a mail message that you wouldn't announce in public. Never count on being able to cancel a message.