Hi
If you go into the spamassassin panel and select the WBL entry option you will find it says:
Input the email address, domain, or IP address that you would like to add to the WBL. Keep in mind that wildcards are allowed in WBL entries. For example spammer@spam.central.com, *spam.central.com, and *central.com are all valid entries
I think that just about explains it.
I use *hotmail.com to block all mail from Bill Gates begging me to come back and use his glorious software so he can have sole admin priviledges over my computer.
John