WWW- When in doubt, leave it out – especially concerning E-mails.

An increasing number of workers are losing their jobs because of E-mail violations, according to an annual survey of about 300 companies released today.

A third of employers in the study sacked staffers in the past year for violating workplace E-mail policies. That’s up from about one in four last year.

“People don’t see a difference between phone conversations and E-mail – but legally, there’s a really big difference,” said Keith Crosley of tech firm Proofpoint, which conducted the survey with Forrester Consulting.

The participants came from public companies, higher education, government and nonprofits with 1,000 employees or more.

Nearly 40% of the employers in the survey have staffers whose job is to read other staffers’ E-mails. And almost half the employers regularly check the contents of the E-mails their people send.

An alarming number of workers don’t realize they’re being watched – and could get in trouble – despite highly publicized corporate scandals.

Prosecutors used incriminating E-mails to help build their criminal case against Enron founder and former CEO Ken Lay – and now he’s facing a sentence of up to 165 years behind bars.

Former Boeing CEO Harry Stonecipher – who had been married for 50 years – was forced to resign after his affair with a female colleague came to light.

The sexually explicit E-mails he sent her – on his company’s E-mail system – became embarrassing evidence.

Former Merrill Lynch analyst Henry Blodget praised stocks in public but derided them as dogs in E-mails – and was barred for life from the securities industry.

“E-mail lives on forever,” Crosley said. “Big companies archive their E-mails – they’re required to, for all kinds of regulatory reasons.”

Employers have a problem with employee blogging, too.

For the first time, the survey queried companies about their policies concerning blogs and message boards – and found 7% of the respondents had fired people in the past year for breaking the rules.

Retailers, wholesalers, telecom firms and utilities were among the most likely firms to fire workers because of E-mail infractions, Crosley said.

Employers weren’t asked exactly why workers got the ax. But Crosley has heard plenty of anecdotes.

Some firms have rules against using the company E-mail for personal messages – and that’s all it takes.

Another likely firing offense is sending messages with “unacceptable content” – such as racist or sexist jokes, or smutty stories or pictures.

Other people lose their jobs because they use workplace E-mail for criminal behavior like stealing customers’ credit card or Social Security numbers, he said.

Employers have good reason to worry about employee E-mails. In the past year, a quarter of those in the survey were ordered by courts or regulators to hand over E-mail records. But the survey suggests they aren’t doing enough to prevent misconduct.

Though more than 80% of these firms have written policies outlining acceptable E-mail use, just half gave workers formal training on E-mail policies in the past year.

Crosley hopes the survey is a wake-up call to managers to educate their employees about acceptable E-mail usage.

Workers should ask bosses or tech departments what the rules are – pronto.

And they should remember Crosley’s rule of thumb:

“At work, don’t put anything in an E-mail you wouldn’t want the whole world to see.”