BATV (Bounce address tag validation) Address validation
Spammers can forge the sender address to look like a legitimate third party email address. The real address then receives bounce backs such as none delivery reports. So effectively the spammer offloads the entire processing load to the real and innocent end user’s email server. They also leave the end user’s confused and left with a job cleaning up none delivery reports and determining why they have received these bounce back messages. A spam proxy server can block illegal bounce back messages. When the original email did not come from the internal network or domains, a spam filter can identify and block these.
This is done by marking each outbound email message with a key and when the bounce back arrives, the proxy server will identify that the key is either present or not present. If the key is not present then the proxy server knows it is not a valid bounce back and so drops the email. This feature is known as BATV.
Wikipedia's guide to BATV