[MetalMilitia]

I guess e-mule is another P2P program so hashing is used to verify large chunks of data.
The data is split into small pieces which are individually hashed then hashed together until one root hash is formed. All of your share will need to be hashed to begin with but once it is done only new files need to be hashed.
One of the good things about hashing the data is that when looking for alternate sources for a download it does not need to look at the file name and just the hash of the file, as indentical files will have an identical hash, this also means the possiblility of downloading a corrupt file form a source is reduced.