A recent update of Google's terms of service has confirmed what many already thought: Users of their popular, free, email hosting service 'Gmail' are actively having their emails scanned for advertising purposes.
As reported in The Guardian
"The revisions explicitly state that Google’s system scans the content of emails stored on Google’s servers as well as those being sent and received by any Google email account, a practice that has seen the search company face criticism from privacy action groups and lawsuits from the education sector."
Google use the information found by automated scanning to build user profiles, which then allow them to display relevant advertisements to email users around the web. Users who are concerned over email privacy should consider switching to a fully-featured email service which doesn't scan messages.