a mini anti spam with lisp

Hello everyone,
I am looking for information that will help me start my project in lisp.
In fact, I must make a mini anti spam lisp that will take as parameter a text file containing an email, then returns if it is spam or not.
I need a very basic algorithm that I have to implement in Lisp.
How can I do?
Thanks.

Here are some pages that discuss the topic.
http://www.paulgraham.com/antispam.html

For any solution, it helps to collect a bunch of "good" messages and a bunch of "spam" messages to train against.

Here's a simple algorithm.

Count the words that appear in each, and calculate each word's spam-factor as

g=(number of times this word appears in a good message) / (total word count of all good messages)
b=(number of times this word appears in a bad message) / (total word count of all bad messages)
spam-factor=(g-b)/(g+b)

Then to score a new message, accumulate the spam-factors for each word in the messages (if a word appears 3 times, add the factor 3 times), and then divide by the number of words in the message. Pick a threshold between +1 (pure good) and -1 (pure bad); messages scoring below the threshold are declared to be bad.

That should give you something to work with. May not work terribly well, though.

thank you very much.
is an interesting approach, I will try to develop it.
but I have another technical problem with reading files.
In fact, I need to make a function with :
input = the name of a file to read.
output = a list containing the data file.
example :
file.txt = { Hi, i'm sorry i can't come at home tonight.}

input = file.txt
output = ( hi i'm sorry i can't come at home tonight )
there ar any way to do like this ?
Thanks.

(with-snark "No, CL is a dead-end language, designed for AI before files even existed.")

Here's a useful reply.
http://www.gigamonkeys.com/book/files-and-file-io.html