Postfix and Dkim

[joemiller]

Hi, I'm running a Centos 6.3 and ispconfig 3 mailserver. I've installed postfix, dovecot and squirrelmail, which are all running fine. I'm trying to get opendkim working with postfix. I have got openskim to generate a dkim signature, which is added to outgoing emails, and also added the corresponding dns entry to match it.
However, when I check the dkim entries with http://www.brandonchecketts.com/ is says:
result = fail
Details: bad RSA signature
This is confirmed by sending emails through my hotmail account, and looking at the source. A quick check here shows that the public and private keys are correct, but they're not getting matched for the message. I'm assuming that the message is getting altered by some process after the dkim header has been added, but I'm at a loss to work out where that's happening, and what to do about it.
Can anyone here shed any light on this problem?
Many thanks.