Create a file /var/qmail/control/dksign containing one line:

/etc/domainkeys/%/default 

The % will be replaced with the domain name in the From: header (or the Sender: header if it exists). If no file exists for the given domain, parent domains will be tried. For example if the message is from foo@bar.example.com, /etc/domainkeys/bar.example.com/default will be tested first. If the file does not exist, /etc/domainkeys/example.com/default will be tested. If no key can be found, the message will not be signed. If a key exists, but cannot be read or contains invalid data, the message will not be sent and will remain in the queue until the problem is fixed.

If you do not create the /var/qmail/control/dksign file, no messages will be signed.

ls /var/qmail/control/dksign