Spammers often regard responses to their messages—even responses like "Don't spam me"—as confirmation that an email address is valid. Likewise, many spam messages contain Web links or addresses which the user is directed to follow to be removed from the spammer's mailing list. In several cases, spam-fighters have tested these links, confirming they do not lead to the recipient address's removal—if anything, they lead to more spam.
It must be noted that sender addresses are often forged in spam messages, so that responding to spam may result in failed deliveries or may reach innocent e-mail users whose addresses have been abused. In many countries providing a false identity in that way is a criminal offense. Criminal spammers sometimes send their messages from purposely compromised computers in order to hide their real identity. Benign spammers reveal their identity, allowing recipients to respond.
In Usenet, it is widely considered even more important to avoid responding to spam. Many ISPs have software that seek and destroy duplicate messages. Someone may see a spam and respond to it before it is cancelled by their server, which can have the effect of reposting the spam for them; since it is not a duplicate, the reposted copy will last longer.
