Sender ID Support in Exchange 2003 service pack

"A significant new feature included in the update is support for Sender ID, an e-mail authentication protocol which Microsoft is backing as a solution to spam against rivals like DomainKeys, which is being pushed by companies like Yahoo and Cisco. Sender ID verifies the Internet address of the server which sent an e-mail against the known IP address of the domain which it claimed to come from. This helps to evade a technique known as 'spoofing' in which spammers send e-mail from fake address to avoid being caught." Source: zdnet.com.au