The way that the Anti-Spam and Anti-Virus Email Filtering System functions is that the MX record for the domain is modified to point to the Anti-Spam and Anti-Virus Email Filtering appliance (cskv.speedingbits.com). The Anti-Spam and Anti-Virus Email Filtering appliance filters that email then forwards the "good" email to the domain's mail server.
Sometimes spammers will disregard the MX record and send email directly to the IP address of the main hostname. You can tell if an email was not processed by the Anti-Spam and Anti-Virus Email Filtering Service by looking at the headers of that email. If the hostname "cskv.speedingbits.com" is not mentioned in the email header then the spammer bypassed the MX record.
This spam can also be blocked if you wish. First, the following criteria must be met:
1) The domain must:
a) Be on its own IP address; or
b) Share an IP address only with domains that also are using the Anti-Spam and Anti-Virus Email Filtering Service; or
c) Share an IP address with domains that do not send or receive email.
2) The outgoing mail server (SMTP) for all email users for this domain needs to be set to:
a) Another mail server, such as their ISP's mail server; or
b) An IP address on their hosting account that does not have SMTP blocked on the server; or
c) A hostname that is associated with an IP address that does not have SMTP blocked on the server.
Once the above criteria is met our technical support staff can set an iptables firewall rule that will block all SMTP traffic to that domain's IP address, except for traffic that originates from the Anti-Spam and Anti-Virus Email Filtering appliance.