BOUNCE_SPAM

This configuration variable determines whether spam messages are bounced back to the sender.

If set to 0, messages containing suspected spam are just dropped without being returned to the sender. If set to 1, they are bounced/rejected. There is a trade-off to be made regarding BOUNCE_SPAM. The default setting of 0 will cause false positives from the spam filter to result in valid messages being dropped without any notification to the sender. Setting it to 1 may cause spoofed email messages to be bounced to innocent third parties. These bounces are perceived by some people as spam and can result in your site being incorrectly blacklisted as a source of spam.

The default value is 0 (spam is simply dropped and not bounced back to sender).