On OS X Yosemite, you may have noticed that Apple's Spotlight search function is rather sophisticated, allowing you to search the web as well as peering into your machine for content too. All very clever.
However, it also has a flaw that could well expose your local information to nefarious types. Not so clever.
So what's going on? Well, the weakness focuses on Apple Mail. Basically, as Spotlight Search indexes emails that have been received within Apple's email service, it also shows previews of your emails, your images and such.
All a hacker would need to do is to insert a tracking pixel into one of your email's images and hey presto! They could well be enjoying access to your data!
While the email is in your inbox, you can ignore scams, but Spotlight's preview function opens up a vulnerability. Seeing as Spotlight opens previews of your junk and spam messages, this could be a problem. Even if you have switched off the "load remote content in messages" feature, it doesn't exactly fix the problem.
Until Apple issue a fix, the best thing for you to do is to go to your Mac System preferences and switch off email indexing.