Exchange 2010 ignoring SPF record

we are using Exchange 2010 and have already setup SPF record for our authorized mail server IPs. As a test using our domain name i tried to send an e-mail from another computer on the internet which has an IP which is not listed in SPF record. Other e-mail services rejected the message due to SPF fail but our own exchange accepted it. This tells me that our Exchange is not rejecting e-mail which fail the SPF record. I enabled SenderID on our exchange to reject e-mail which fail senderID but made no difference.

Below is the SPF info i pulled out from the message header which failed SPF but got accepted by our Exchange.

Received-SPF: None (exchange2010.domainname: user@domainname does not designate permitted sender hosts)

If your Exchange server is honoring SPF records for other domains but not yours, then you probably have your internal DNS set to be authoritative for your domain name, but you don't have the SPF record in your internal DNS servers.

Solution: Add your SPF record to your internal DNS servers.