If you read the article linked here, the law was changed in 2009, partly as a consequence of the 2007 scandal in which some Bush emails weren't preserved becaus

If you read the article linked here, the law was changed in 2009, partly as a consequence of the 2007 scandal in which some Bush emails weren't preserved because they were run off of a server managed by the RNC. The agency must make sure that emails are being retained according to law. In any private enterprise that must retain records, there is exactly one way to do this: you use your employer's email server, which has a long audit trail and dedicated compliance officers.