BackScatter consists of Non-Delivery Reports (NDRs), otherwise known as bounce backs, for e-mails that a given user did not send. This means that the email address was spoofed and spam was sent out using your domain. Of course, a large portion of the spam that was sent out was sent to email addresses that do not exist.
So if the address does not exist, now what happens? The user [you] will get a bounce back for an e-mail message your did not actually send. The bounce back your receive is considered backscatter.
Spammers almost always spoof someone’s address when sending out spam, so this means that almost all spam looks like it was sent from someone else. As such, if someone’s address gets spoofed and a million e-mails were sent out using your address, half of which were sent to addresses that don’t exist, you are going to get a lot of bounce backs.
There are tools on the WestHost servers to try and prevent backscatter, but please contact Support if you have questions.