SpamCop FAQ : Help for abuse-desks and administrators : Robots: Mailing lists and autoresponders :
Your server is basically a special kind of relay. Your ML software must preserve the headers of the source email the same as any other relay. If your software trashes the headers, then you will continue to be victimized by spammers because you are effectively anonymizing their email - just what they want. See the relay section at Open Relay Servers, on correct relay behaviour.

Also, I have created a special set of FAQs for mailing list administrators (in this section of the FAQ). It includes info on how to reduce spam complaints and handle the ones you do get gracefully.
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