Email backscatter occurs when a spammer spoofs a legitimate domain to send a spam email to a non-existent recipient on your server. Since the email cannot be delivered, your server sends back a Non-Delivery Report or NDR (also known as a "bounce message") to that domain, who initially had nothing to do with the original email that was sent.
This is a problem, because your server will flood that legitimate domain with NDR emails, putting extra load on them, and yours, spamming them with NDR reports for emails they never actually sent.
To prevent this, you can use SPF, DKIM, and DMARC.
To learn more about SPF please click the button below.
To learn more about DKIM please click the button below.
To learn more about why DMARC is needed please click the button below.