Exchange Server 2010 SP1 provides "ability to disable algorithms which're not FIPS 140-2 compliant; these algorithms are used for encryption, hashing, and signing within Windows Server 2008 and 2008 R2 OS that support Exchange 2010. When Security Options local policy or group policy setting for System cryptography: Use FIPS compliant algorithms for encryption, hashing, and signing is enabled this disables the use of non-FIPS, such as RC-4, and caused certain functions within Exchange 2010 RTM to fail. The most notable issue was in Outlook Web App as documented in KB977961 and web-based Exchange Control Panel (ECP)," explains Microsoft.
FIPS is an acronym for Federal Information Processing Standards (FIPS); this's a standard utilized to define security and interoperability requirements for cryptographic algorithms US Government uses. FIPS 140-2 Publication and standard defines cryptographic algorithms as well as standards for key generation and key management.