The project Immersion of the MIT Media Lab (it is an email metadata visualization tool) translates metadata from your email into a map that displays your relationships with recipients or recipients of your emails.
If you do a test you will be scared. Your metadata can tell you a lot about you. Try Immersion for your Gmail and will visualize all of your online relationships in the last 10 years.
The app scans only the From, To, Notification fields. and the date of the messages in Gmail, Yahoo Mail, or MSExchange. Scanning my Gmail account showed how many messages I have received in the last 10 years I use the service. The app detected how many partners I have and could predict how many new partners I have by the end of the year and how many messages I will send and receive the rest of 2014.
To test with your email, you must grant access to the application to scan everything that we described above.
Email metadata is easy to see, but it's very difficult to delete them. The MakeUseOf Guy McDowell explains how you can see the header information in Gmail messages, and how to decipher the metadata associated with your emails.
You can now imagine the size of each user's exposure to the internet. If you think that each file contains metadata (Office files, PDFs, photos) you can understand what a government or a malicious user can know about you.
