Allowed prefixes for the approver e-mail address are:
Furthermore, the domain part of the approver e-mail address must be identical with the domain which you want to create a certificate for. I.e. for a certificate order for domain.com the approver e-mail address must be email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org or email@example.com.
Next to these standard e-mail addresses it is possible to use a different e-mail address. However, this requires that the e-mail address is available in the Whois for the respective domain. Under the GDPR many registrars - including Key-Systems - will not publish e-mail addresses in Whois per default. As a consequence a Certificate Authority (CA) is not able to check if a given approver e-mail address is listed in Whois of the respective domain. The mentioned standard approver e-mail addresses will always work, regardless of Whois. Thus we always advise to use one of these standard e-mail addresses or use the more automated approval methods via HTTP and DNS.