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
#Socket inet:8892@localhost
Socket local:/var/run/opendkim/opendkim.sock

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.