Issue was noted with a Virtualmin server running on Ubuntu 18.04. Already setup the DKIM settings from the control panel, the DNS records have been propagated too, but the outgoing mails are not getting signed with DKIM key.
Seems to be an issue with the opendkim setting
Open /etc/opendkim.conf and check for the setting Socket
Uncomment this “#Socket inet:8892@localhost” and comment the line which directs to opendkim.sock.
So finally, the setting should be like :
Socket inet:8891@localhost #Socket local:/var/run/opendkim/opendkim.sock
Now, restart the service opendkim
The mails should be now signed with DKIM.