A Redundant File is any file that was previously transmitted via an Advanced Outbound Rule and no longer exists at the source location.
Using the Clear Redundant Files option instructs a recipient MailRules to permanently delete these files from the destination.
When Clear Redundant Files is enabled, a record is maintained of all files that have become redundant and this information is transmitted with each send of the Outbound Rule. By storing this information, if you add recipients to a rule, you can be sure that any files that became redundant (however long ago) will still be removed at the destination. However, if files become redundant on a regular basis, this does mean that a large record of redundant files may be maintained and is always being transmitted.
In an Outbound Rule, next to the Clear Redundant Files option, is the Information button. Clicking the Information button will tell you how many files are in the redundant file list and you can choose to clear this list.