The cause of this error is most likely due to having more than one SPF record on your DNS.  You might want multiple outbound mail servers to be authorized to send messages from one domain.  However, multiple SPF records are not recommended and can cause delivery issues.  To mitigate this, instead of having two SPF records, use the "include" syntax to have both mail servers on one record.  For more info on correct configuration of your SPF records, see http://www.openspf.org/.