"It can cause a lot of trouble from an identity theft perspective," he said. According to the Internet Crime Complaints Center, he said, individual fraud cases are up about 22 percent, from about 285,000 cases in 2008 to 336,000 cases in 2009. While not all of those cases are e-mail-related, he said they comprise a substantial chunk of the total.
"There's a lot of hassle from a personal perspective," not only in terms of fraud but also in terms of the emotional toll of unwittingly reaching out to people with whom you're no longer in contact, he said.
"If you have ex-boyfriends' and girlfriends' addresses, you could be sending malicious email to exes [and] others you don't want to be involved with anymore," he said.
If you want to keep your computer and personal lives free of complications, experts say the most important piece of advice is to read your electronic mail with a healthy dose of skepticism.
"What arrives in your inbox isn't really that much more secure than what arrives in your mailbox," said Richard Wang, U.S. manager for the Burlington, Mass. security firm SophosLabs. "You should treat it with the same sort of skepticism that you treat whatever arrives in your mailbox every day."
Some of the more prevalent e-mail viruses circulating these days are masked in messages intended to pique your interest, he said.
One common virus is carried in an e-mail purporting to be from the U.S. Postal Service or a delivery company alerting you to a package delivery. Another popular virus tempts victims with an e-mail claiming to be from a job applicant.
But he warned, unless you're sure of the sender, it's best to not let down your guard, especially since it's estimated that cybercriminals generate about 80,000 new malicious threats every day.
"One thing to remember is curiosity killed the cat," he said.