Extract from DESCR:
Deliver allows any user to write a shell script that processes all
incoming mail messages for that user. The system administrator may
also install scripts that process all messages.
The output of a script is a list of mail addresses, files and programs
that should receive the message. It has access to each message as it
is processed, so the action can be content dependent. The script may
also generate automatic replies, like the "vacation" program, or pass
along a modified version of the original message.
