Can email be used for DigiCert Code Signing organization validation?

No.

Organization validation for DigiCert Code Signing certificates can only be completed via phone call. The Certificate Authority (CA) will perform validation by calling the phone number listed in your organization's official registration records; organization validation via email is not supported.

To speed up the validation process, we recommend confirming the availability of your registered organization phone number in advance and ensuring someone is available to answer the call.