The FBI eventually discovered that the emails received by Jill Kelley, a close friend of the Petraeus family, were sent by Paula Broadwell. And as they dug deeper, the affair between Broadwell and Petraeus came to light. NBC's Andrea Mitchell reports.
By Allison Linn, NBC News
There?s a valuable lesson everyone can learn from the scandal involving CIA Director David Petraeus: Take a deep breath before you hit that ?send? button.
The common link in the complex and still-unfolding scandal involving Petraeus, who resigned last Friday, and several others is email ? lots of messages, some now alleged to be inappropriate.
For many workers these days, email is the primary mode of interaction with staff, bosses and clients. Experts say the constant back-and-forth means it?s all too easy to go from an informal exchange to something that could easily offend.
?That?s how we communicate, and it can get out of hand,? said Pamela Eyring, formerly the Chief of Protocol at Wright-Patterson Air Force Base and now the president of the Protocol School of Washington.
Email also has another potential flaw for people who like secrets: It?s not necessarily as private as you might think.
?It can, and will, come back to haunt you,? said Barbara Pachter, also a business etiquette expert.
It was an investigation into allegedly threatening emails that eventually led investigators to Petraeus' biographer, Paula Broadwell. Multiple government and law enforcement officials tell NBC News that the emails revealed the two had been engaged in an extramarital affair.
The person who received those emails, Jill Kelley, has now been swept up in the investigation herself.
The woman who triggered the investigation that led to the resignation of CIA chief David Petraeus threw lavish parties for top military brass ? and also racked up debt. NBC's Kristen Welker reports.
Defense Department officials have said that Gen. John Allen, commander of the International Security Assistance Force in Afghanistan, is under investigation for sending potentially inappropriate emails to Kelley, a Tampa, Fla., socialite.
One reason email is so popular is because it?s so quick and simple. But Eyring, the etiquette expert, noted that can be its Achilles heel. Americans are already an informal people, and the ease of email can mean that communications get too friendly too quickly.
?We get lazy. We cut corners,? she said.
That can lead to more humdrum problems, like embarrassing typos and costly errors.
Pachter recalled a job candidate who dashed off a quick, and typo-filled, thank you note to a potential employer from her mobile phone. She didn?t get the job.
Pachter, who also has been an adjunct faculty member at Rutgers, said she?s scolded students for sending her informal emails as part of class assignments. She advises clients to use salutations, watch your choice of words and double-check your facts in all email correspondence.
Also, double check your ?to? box ? another big area of embarrassment involves sending an email to the wrong person.
These problems didn?t really exist two decades ago. Pachter noted that there was a time when people could send a letter and be reasonably assured that the only other person who would see it was the person it was sent to.
These days, it?s all too easy for employers to snoop through past emails for evidence of inappropriate behavior. From there, it?s often not too long before the private exchanges are being shared with the world, thanks to the Internet and social media.
?People want to keep their private life private. We all want that,? Eyring said. ?But if it?s in an electronic format like an email or a text, it can be shared.?
That includes with investigators. The Equal Employment Opportunity Commission, which investigates workplace complaints, said email in some cases makes it easier to suss out wrongdoing.
?It supplies endless evidence,? said Peggy Mastroianni, legal counsel for the EEOC.
That doesn?t mean that every friendly email to a co-worker will result in a harassment complaint. The EEOC said harassment has to be based on sex, race or religion, and it has to be unwelcome. That means that there has to be evidence that someone in the workplace was unhappy with the comments a colleague made.
Still, that kind of electronic banter can be a slippery slope.
Ray Peeler, a senior attorney in the EEOC?s office of legal counsel, said it?s almost always the case that the initial comments aren?t perceived as so bad, but the behavior becomes more unwelcome as time goes on.
Clearly, it?s important not to cross the line from friendly to inappropriate. But, experts also caution that you may never get ahead at work if your emails are too strait-laced and professional.
Laura Kray, a professor of management at the University of California, Berkeley, has done research showing that playful and platonic flirting can help in business negotiations.
Kray said that kind of flirting involves being confident, positive and full of energy. It?s different from overtly sexual behavior, which she said other researchers have found to be detrimental to women?s careers because it?s seen as compensating for a lack of ability.
Women can be effectively flirty in email if they use the occasional emoticon or exclamation point to show their enthusiasm, Kray said.
?It?s about ? letting your emotional self kind of show,? Kray said. ?If you?re just all stuffy (and) all business all the time then ??particularly if you?re a woman ??then you?re going to be labeled the cold fish.?