US senator Liz Figueroa of California has launched the legal bid arguing that the way Gmail will scan people's emails to target ads is too intrusive.
The challenge follows two other potential hurdles as data protection groups objected to the intrusion into personal email content, while another UK firm claimed it owned the rights to the Gmail name.
Gmail is set to shake up the free email market by offering users more than 100 times more storage space than its rivals. It plans to pay for all the free space it is offering by selling highly targeted ads, which scan incoming email.
It uses similar technology to that used by Google in its search engine to present users with targeted ads when they enter search terms.
The Gmail service works so that should a user, for example, happen to mention in an email that they were looking for a new mortgage, Gmail would hit them with an array of financial ads.
Senator Figueroa has compared the service to having "a massive billboard in the middle of your home".
Privacy groups Privacy International and Bits of Freedom have filed a complaint with the UK Information Commissioner objecting to Google's ad plans for Gmail. Google has said that it will do whatever is necessary to ensure that Gmail complies with data protection rules.
How Google will deal with the threat to the Gmail name is unclear after AIM-listed independent investment research firm The Market Age claimed ownership of the Gmail tag.
The firm said it was seeking legal advice on protecting the name of its own Gmail online product after Google released details of its free email service last week.
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