Hi @andersonvm
This depends on the software you use for Server management such as Cpanel, Virtualmin etc. Since it is free, I installed and use Webmin/Virtualmin.
Some software packages should have been installed in order to use these features. Since I have only one server, I installed all on the same server. Email signing has several components.
I remember that I have installed BIND DNS, DNSSEC, OpenDKIM. Then I configured them step by step.
In virtualmin, you may configure DKIM record, Virtualmin → Email Settings → DomainKeys Identified mail after setting up dmarc records in DNS
Webmin → Servers → BIND → DNSSEC Verification → DNSSEC Enabled
DMARC record is added among DNS records by Virtualmin automatically.
DKIM private key and Public keys are generated through Virtualmin → Server Configuration → Domainkey Options.
You should also visit Virtualmin → Server Configuration → DNS options in order to enable SPF record, DMARC record and DNSSEC signature for each domain.
I also removed the server ip from spam lists because it has been used for spam previously and blocked by several. servers.
Regards,