DKIM is a purely optional procedure for digitally signing emails. cPanel supports this, but this must be activated manually.
To do this, a corresponding DKIM key must first be created for the domain in cPanel under ‘Email deliverability’
The string that you find when clicking "Manage" next to the domain name, then has to be saved as a TXT record for the subdomain
default._domainkey
of the respective domain.
Once the DNS record has been propagated, the signing can then be activated by opening up ‘Email deliverability’ again in the cPanel.