Today I received an email from a person who announced they wished to be anonymous, and didn't want to reveal which organization they worked for. They used Hotmail and a pseudonym to avoid revealing their identity, and asked some detailed questions. That left me very curious to know how I was replying to, so I checked the message headers and they contained the IP address of the computer they were on. Running whois on that IP gave me the company they worked for, since they were apparently logged in from a work machine.
I'm not going to go into details on exactly how to do this sort of detective work, instead I want to focus how to fix prevent information about your location leaking into your email headers. The main culprit are headers that show the IP address of the original machine that the email came from. Here's an example that came from someone logged into Yahoo through a browser:
Received: from [18.104.22.168] by web50009.mail.re2.yahoo.com
And here's someone who emailed from Hotmail's website:
If you use a desktop program like Outlook or Apple Mail with any account, the IP address of your machine is almost always included in a header that looks like the Yahoo example.
Why should you care? That IP address will pinpoint your organization if you're within a company, or your ISP and a rough location if you're using broadband from home. If you're working on a side-project you want to keep separate from your employer, and they get hold of your sent emails, that header is proof that you were using work equipment on your idea and potentially gives them ownership when your startup becomes the next Google. And if your email with a doctor's note has an IP address in Cancun, you may have some questions to answer! (I actually ran across this flaw when I was looking at matching email contacts with other accounts, using geolocation on the IP address to figure out if it was the John Smith in Denver or LA, but I decided that was too creepy)
What should you do? The simplest fix is to use Gmail. As far as I can tell they're the one mainstream provider that doesn't include the IP address in the headers. The Chinese hacking incidents show they're not a panacea for all your security problems, but they definitely seem to have got this right. There's a lot of other more complex techniques that could safeguard your privacy, but if I was recommending something to a family member, I'd go with Google. You do need to be careful that you log into the website interface when you want to send an anonymous email though, since desktop programs tend to add the IP address anyway.