Up to date Virtualmin on up to date Debian 9 : Webmin version 1.883 Usermin version 1.741 Virtualmin version 6.03
All outgoing mails are signed whatever the from domain is.
This is because the Domain directive in opendkim.conf is beign ignored as states the opendkim.conf (5) man page :
Domain (dataset) A set of domains whose mail should be signed by this filter. Mail from other domains will be verified rather than being signed. This parameter is not required if a SigningTable is in use; in that case, the list of signed domains is implied by the lines in that file. This parameter is ignored if a KeyTable is defined.
Additionnaly all emails have a double signature. But this is another issue and I couldn't find why at the moment.
Best Regards, Xavier