can't determine message sender; accepting

From: Ricardo Fraile <rfrail3_at_yahoo.es>
Date: Wed, 5 Dec 2012 12:09:49 +0000 (GMT)

Hello,     I receive "can't determine message sender; accepting" error when i send a message outside the same server where open dkim are running, but it i send the same mail inside the same server, i get the dkim header. why? Opendkim Version: 2.6.8-2~bpo60+1 Thanks,
Received on Wed Dec 05 2012 - 12:10:04 PST