When you use start the Move Mailbox wizard you get quite a few options, like skipping the corrupted messages, and which Active Directory directory servers to use. But you also have the option to exclude rule messages, only when the target database is Exchange 2003 or Exchange 2000. When you select this option, the rule messages will not be moved. The reason the option is available is due to the change in maximum rule quota limit. In Exchange 2003 and 2000 this limit was 32KB, in Exchange 2007 it is set by default to 64KB, but can be easily changed using the Exchange Management Shell to 128KB or 256KB (Set-Mailbox -RulesQuota:. Therefore, if you move a mailbox from Exchange 2007 to Exchange 2003/2000 the move might fail if the size of the rules are higher than 32KB, and that's why you can choose not to move the rules.