google and privacy

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I have been a user of gmail for a long time and i have been using it because it's very user friendly and fast .But i get a feeling that google goes through our data everytime we see or send a mail,and accordingly some ad line comes up just above the box.it's ok if it advertises but it's going through our mails and correspondingly giving ads,which i personally feel isn't right.how do u people feel about it??