What kind of spam protection is used on your servers?
Availability of the sender's IP address'es in the RBL lists is checked (spam-lists of known organizations Spamcop, SpamHause). Sender is checked on the servers, meaning the mail will not be accepted if there is no sender's mailbox on the server from which this email came from.
You can also configure anti-spam filter SpamAssassin yourself, in the administration panel cpanel