"Google also places advertising on Gmail based on key words that appear in messages transmitted through our system (it’s a good example of ads helping to pay for the free services we all enjoy online) - so if you’re emailing a friend about a trip to Paris, for example, ads might appear on the right hand side of the page for trains to France. Google does this using software similar to the kind that scans emails for viruses, to filter out spam and turn the bits of data received into the characters on the screen. No human being other than the user ever reads the messages sent or received on Gmail – it’s simply a computer matching up key words in peoples’ emails with targeted ads." -- https://static.googleusercontent.com/media/services.google.c... (~2008)

(Disclosure: I work for Google, speaking only for myself)