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Files and programs are legitimate message recipients. Files provide archival storage of messages, useful for project administration and history. Programs are useful as recipients in a variety of situations, for example, to maintain a public repository of systems messages (such as the Berkeley msgs program).
Any address passing through the initial parsing algorithm as a local address (not appearing to be a valid address for another mailer) is scanned for two special cases. If prefixed by a vertical bar (``|'') the rest of the address is processed as a shell command. If the user name begins with a slash mark (``/'') the name is used as a file name, instead of a login name.
Files that have setuid or setgid bits set but no execute bits set have those bits honored if sendmail is running as root.