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. 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.
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