via Erik Larkin, PC World...
Your Web mail account is a treasure trove of private and potentially valuable information -- and thieves know it... Normally you can't tell whether you've been hacked... Even if you cannily leave a juicy-sounding e-mail unread, a thief or snoop may read it and then return its status to unread. But with a little bit of know-how, you can create an electronic trip wire that will trigger whenever someone reads a rigged e-mail... The gist of it is to keep an e-mail message in your account that includes the code for a counter. Opening the attachment trips the counter, thereby alerting you that someone was snooping. (
how-to details)
