Google is suing the US government over claims that the search company was excluded a Department of the Interior (DOI) request for proposals to provide a hosted e-mail service.
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The litigation comes a year after Google launched a campaign publicising organisations switching to Google Apps, marking increased rivalry with Microsoft.
According to Google, the request for quotations (RFQ) for the contract - estimated to be worth $59m over five years - specifies that only the Microsoft Business Productivity Online Suite-Federal could be proposed.
Google's lawsuit against the federal government alleges Microsoft's success in the bidding process was pre-ordained.
The DOI's chief technology officer, William Corrington, allegedly told Google there would be no opportunity to compete because its product was not compliant with DOI's security requirements.
But, Google claims that the DOI declined to provide those security requirements or meet company representatives to discuss Google Apps security.
In its complaint, Google argues that - while Microsoft topped a list of 12 major software security providers for the number of security vulnerabilities and software patches needed to plug security holes - Google was the only provider on the list with no zero-day disclosures.
All parties involved have declined to comment on the pending litigation, the US reports said.