If you're a worker who spends a fair amount of time sending emails while at work, the evidence is mounting that you should take two breaths before hitting "send."

Almost one-third of companies said they've fired an employee in the last 12 months for violating email policies and 52% of the companies said they have disciplined an employee for violating email rules in the past year, according to a survey of 294 U.S. firms with 1,000 or more workers.

Thirty-eight percent of companies said they employ staff to read or analyze outgoing email messages, and that jumps to 44% of companies with 20,000 or more employees, according to the survey, conducted by Forrester Consulting for Proofpoint Inc., which makes anti-spam and email monitoring tools for companies. About half of the companies said they regularly audit outbound email content.

"There are legitimate reasons for companies to monitor email," said Keith Crosley, director of market development at Proofpoint, in Cupertino, Calif.

"There is so much risk associated with email. The companies we're talking abut here have records on many thousands of customers. You really need to protect that data. email is one of the least secure systems," he said. "It's very easy to inadvertently reveal massive amounts of customer data."

The message to workers is "don't put anything in emails that you wouldn't want the whole world to read," Crosley said. "That would be a difficult ideal to live up to," he said, but, absent that, workers should at least abide by their company's policy for acceptable email use.

What email policy?

Of course, you might be forgiven for not knowing your company's email policy. While 83% of companies said they have an acceptable-use email policy in place, just 51% of companies said they'd trained workers on their email policy in the past 12 months, according to the survey.

"It's likely that your company doesn't perform specific training on those," Crosley said.

While it's important workers consider their company's email policies before forwarding yet another of those messages with a silly joke, most companies are more concerned about email messages that divulge confidential data or violate regulations.

When asked about their concerns regarding outbound email messages, 71% of firms said they're concerned or very concerned about protecting personal identity and financial information, and the same portion of companies are concerned or very concerned about complying with corporate-governance regulations.

About 66% of companies are worried about preventing leaks of confidential memos, the same portion pointed to protecting private health-care information, and 66% also said they're concerned or very concerned with making sure company trade secrets don't get sent out via email.

Meanwhile, 62% of companies pointed to "internal email policy compliance" as a concern, and 54% said they were concerned or very concerned with "monitoring for inappropriate content."

"People often key-in on people sending porn, people wasting time, but there's a significantly higher level of concern around protecting financial [and preventing other] information from being divulged inappropriately," Crosley said.

"As a manager, I am not as concerned about Bob in accounting doing a little bit of personal email, because our work lives and our home lives are converging. People have mobile devices. They're at the office all the time, even when they're away. An enlightened policy would allow a certain amount of personal email use."