11. A system for processing email messages comprising: a processor operable to execute computer program instructions; a memory operable to store computer program instructions executable by the processor; and computer program instructions stored in the memory and executable to perform the steps of: receiving email messages in a local inbox or in an inbox on an email server, or both, the received email messages having been scanned using spam detection rules to determine whether or not they include spam; obtaining updated spam detection rules; and rescanning at least some of the received email messages using the updated spam detection rules to determine whether or not they include spam; wherein each email message includes information indicating a version of spam detection rules that were used by the email server to scan the email and the rescanning is performed by rescanning only email messages that were scanned using a version of spam detection rules that is older than the updated spam detection rules, based on the information.
11. A system for processing email messages comprising: a processor operable to execute computer program instructions; a memory operable to store computer program instructions executable by the processor; and computer program instructions stored in the memory and executable to perform the steps of: receiving email messages in a local inbox or in an inbox on an email server, or both, the received email messages having been scanned using spam detection rules to determine whether or not they include spam; obtaining updated spam detection rules; and rescanning at least some of the received email messages using the updated spam detection rules to determine whether or not they include spam; wherein each email message includes information indicating a version of spam detection rules that were used by the email server to scan the email and the rescanning is performed by rescanning only email messages that were scanned using a version of spam detection rules that is older than the updated spam detection rules, based on the information. 12. The system of claim 11 , wherein the received email messages were scanned by the email server.