[exim] DKIM for specific virtual domains

Hi,

I'm implementing domain keys for the first time on Exim and have
updated the remote_smtp section to this:

remote_smtp:
driver = smtp
dkim_domain = ${sender_address_domain}
dkim_selector = dkimxy
dkim_private_key =
/usr/local/etc/exim/${sender_address_domain}/dkim.private.key
dkim_canon = relaxed

I was just wondering what the recommended config would be to have DKIM
only used on specific domains. One option is to simply have no domain
private key for domains without DKIM and let Exim fail reading the key
and send anyway (this will log an error everytime to the mainlog), but I
guess there must be a cleaner way to do this.
Can anyone advise?

thanks in advance, Andy.