From this message it seems that the SMTP server where you're forwarding the email to is performing SPF checks for spam. Please note that this should not be done, as those checks will likely always fail. SPF is used by the sender's domain to specify which IP addresses are allowed to send emails on their behalf. When SpamFilter forwards an email to your destination SMTP server, your destination SMTP server sees SpamFilter's IP address as the sender of the email, not the original IP that originally delivered the email to SpamFilter. If the email is valid, then the SPF check performed by SpamFilter will validate the sender's IP and pass on the email to your destination SMTP server. However if your destination SMTP server performs another SPF check, it will see SpamFilter's IP address as the sender and will reject the email, since SpamFilter's IP will unlikely be in the SPF record for the sender to authorize it to send emails.

This said, if the email was spam, and the SPF record for the sender was configured properly, SpamFilter should have rejected the email if you had the SPF filter enabled and the email was not whitelisted for some reason. If you can zip and email us SpamFilter's activity logfile (possibly trimmed to the 10 minutes before and after 05.25.12 13:18:51:722 at which the email was received) we can go thru it to see what happened.