You’ve configured opendkim for your smtp server but when clients use your smtp servers emails are not signed.

By default opendkim trust only local host, you have to specified all the host to trust.

Uncomment the following line in opendkim.conf:

# InternalHosts   refile:/etc/opendkim/TrustedHosts

And inside this file specify the hosts you trust:
<CIDR> # all kind of CIDR for instance:

Reload opendkim service

systemctl reload opendkim

It is important to note that the same mechanism is used in postfix via the mynetworks parameter in (for instance: mynetworks =