The term list poisoning refers to poisoning an e-mail mailing list with invalid e-mail addresses.

Once a mailing list has been poisoned with a number of invalid e-mail addresses, the resources required to send a message to this list has increased, even though the number of valid recipients has not. If one can poison a spammer's mailing list, one can force the spammer to exhaust more resources to send e-mail, in theory costing the spammer money and time.

Poisoning spammers' mailing lists is usually done by blacklists submitting fake information to email submit style offers, or by posting invalid email addresses in a Usenet forum or on a web page where spammers are believed to harvest email addresses for their mailing lists.