Having a company wide email system has many advantages and disadvantages. The bottom line of this system, is that it is for the employees of that company to stay in contact with one another more easily. Sending information to key personnel about meeting changes, sending out updated dress code guidelines about the up coming casual Friday, or simply to pass a long a "Good job" from a boss to an employee, the company wide email system is just that, the company's. Blatant misuse of this service could find you in hot water with your supervisor or co-workers.
Much like sending a letter through the mail, or passing a note to a schoolmate when you were younger, the email can be very helpful or harmful. Emails can be tracked by the company you work for and used against you when promotions or raises are awarded. The opposite is true also, although hardly ever utilized in that way.
When sending emails containing disparaging remarks about a co-worker, supervisor, or the company to other people, the company can use those emails as cause for your termination. Emails can be saved for future use, forwarded to another person i.e. the co-worker or supervisor, or simply deleted. Emails my be electronic in nature, but they leave the same type of "paper" trail as regular mail. They can be traced back to the original sender as easily as looking at the return address on an envelope.
Sending dirty or lewd jokes to a co-worker may seem harmless in and of itself, but if a third co-workers reads that email and is offended by your sense of humor, a sexual harassment charge could be filed against you. A sexual harassment charge filed against you at a place of employment is one of those things that does not have to be proved. Many times, even the hint of harassment is enough for you to be fired. It is easier to just remove the offending employee than it is to investigate the claim.
When using the company wide email system, it is always best to keep all correspondence in a professional way. Thus reducing your chances of termination for inappropriate behavior. When writing emails to co-workers, a good rule of thumb is this: Write your email as if your supervisor were looking over your shoulder. If you would not want your boss to see what you are writing, then you should not write and send the email. Good company email etiquette will go a long way in securing your future with your company.