If you're not careful, your e-mail can get you fired.

Think that's a remote possibility? Think again. At least 25 percent of all companies have canned a worker just for violating the firm's e-mail policy. Most corporate policies regarding e-mail are fairly simple and straightforward and usually grounded in good old common sense.

PCWorld reporter Daniel Tynan has identified seven types of e-mail messages you should never send from work if you want to stay employed:

1. Sexually explicit messages
No matter how great your Saturday night date, don't recount the details in an e-mail to anyone if you're using your work address. Just as the date was personal, keep the details personal and use your personal e-mail address if you must brag.

2. Scam spam
We're all familiar with that message from a deposed citizen of an African nation who needs your urgent assistance so you can become a million dollars richer. No matter how humorous or tempting you may find the offer, do not forward it to friends or colleagues.

3. Pornographic photos
Dow Chemical. Hewlett-Packard. The New York Times. Do you know what these three companies have in common? They have each fired employees for sending e-mails to their work buddies with Web site links to porn photos. Is your job and family's security worth that?

4. Messages denigrating the boss
Here's a fun fact of corporate life of which you may not be aware: Your company probably has a system in place to monitor your e-mail. Complain about the boss and chances are very high someone you really don't want to read your tirade will see it--your boss. Vent verbally. That's why there's a water cooler!

5. Jokes
Your colleagues may not share your sense of humor, especially when it comes to off-color, sexist, or racist jokes. True story: After Chevron employees passed around an e-mail titled "25 reasons why beer is better than women," four female employees sued the company for sexual harassment. The cost to Chevron: a whopping $2.2 million. Not much of a joke, after all.

6. Sensitive, personal information
Hey, bosses: When it comes time for your employees' quarterly and annual reviews, do it in person and not by e-mail. When you use e-mail to communicate an employee's performance evaluation, salary package, and Social Security number that can become part of the public record should the company be investigated or get involved in a legal action.

7. Messages urging someone to break the rules
Breaking the rules is unethical. Breaking the law is illegal. Either way, don't ever send an e-mail urging anyone in your company to do anything illegal, such as destroying files. Exhibit A: Investment banker Frank Quattrone, who was sentenced to 18 months in prison for obstruction of justice when he sent an e-mail to Credit Suisse First Boston employees encouraging them to destroy evidence requested in an SEC investigation.