Docs could be better, but once installed it's pretty dead simple. How it works is by simply monitoring the spool folder for new mail. Before SM can process the message USM grabs it checks to see if it matches any rules, modifies it and puts it back in the spool folder for processing by SM. The rules/actions are adequate, but I've seen better ( Exclaimer, CodeTwo, etc ), but modifying the Message Header is one of the things that can be done. The only thing that is really missing is Tokens in the "replace" part of the actions.