The LinuxRulz mailserver network employs a form of fighting spam called Sender Policy Framework (SPF). This method allows us to verify the email we receive is from a valid source, valid sources of email for a particular domain are published by the senders' domain administrator, not us. For mail to be rejected by SPF means the senders' domain administrator has explicitly requested this to happen, we are just acting on this request.

If you are having trouble receiving mail and the error returned to the sender contains "Sender address rejected: Please see http://spf.pobox.com/why.html?sender=XXXX&ip=YYYY&receiver=ZZZZ : Reason: mechanism", the sender must contact his or her ISP with the error to find out why they cannot send email. To re-iterate, if the email is rejected, the domain administrator of the senders domain has explicitly requested us to verify if the email comes from a valid source, if not to reject it, valid sources are published by the domain administrator, not us.

Official SPF Site
How SPF Works
Information for Users