You should never, ever send a credit card number over email unless you have encrypted it in a reliable way. The same goes for your bank account numbers, social security number, and even your driver’s license. See Send Secure Info Over the Internet for ways to safely transmit this type of information.
Since you’ve already made that mistake, should you cancel your credit card? That might be an overreaction. Call the credit card company and discuss it with them. If they say it’s not necessary, believe them. If someone hijacks your card, they’ll lose more money than you will.
I actually had an experience like this a few years ago. No, I didn’t email a credit card number. A new clerk at my synagogue sent me an email that included my credit card number. I immediately called the bank. They told me they’d keep an eye out for suspicious activity, but that cancelling the card wasn’t necessary.
I’ve paid my dues by check ever since.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at answer@pcworld.com, or post them to a community of helpful folks on the PCW Answer Line forum.