Emails either sent to a Gmail account by both GMail and non-Gmail subscribers are browsed to lift any company names or other content which can then be used as "ad targets". This is from their own FAQ section
"1.1 What is Google's Gmail?
Gmail is a web-based e-mail service offered by Google offering one-gigabyte (1000 megabytes) of e-mail storage to users, five hundred times the capacity offered by Microsoft's Hotmail (as of June 2004, although banner advertising-supported web-based e-mail services, including Hotmail and Yahoo! have or plan to increase their storage capacity in response to the competitive pressure of Gmail).
Gmail is supported by advertisers who buy keywords, much like the Google search engine's AdWordsadvertising program. Gmail uses "content extraction" (the term used in Google's patents) on all incoming and outgoing e-mail in order to target the advertising to the user. For example, if the user is having an e-mail conversation about applying for a job, Gmail might present the user with ads about online job search sites and resume writing services."
This doesn't mean that anyone can hack in any more easily than any of the other email providers though - it's just that your emails are searched by computer for the relevant words.
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