Stop Spam emails been sent from my server. by allsopp2k7

I have my own server and somehow someone or something is sending 1000s of emails from it! I get 1000s of undelivered emails each day postmaster ones. Your job will be to not only stop the spam emails, but to find out how and why they was coming and to secure it so it dosent happen again... (Budget: $30-250, Jobs: Apache, PHP)
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