It is the first of 15 agencies that are planning to move at least part of their e-mail to the cloud, GSA administrator Martha Johnson said in a blog post.
Johnson estimated that the GSA will save $15.2 million in the next five years because of the move, which she said will help the agency become more mobile and efficient.
Google, Microsoft and Amazon are all prominent companies pushing their cloud services to businesses and the government, touting the convenience and making assurances about security.
E-mail may be the easiest service to sell for these companies, which could have a hard time overcoming worries about cloud safety with more sensitive content.
The Obama administration is pushing a “cloud first” policy, encouraging federal agencies to move at least three services to the cloud. The federal government announced the plan in November 2010.