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From the Configure Mail Workflow screen, click Add/Remove link under the Mandatory Recipients heading next to the required mail type.
Select whether the mandatory recipients for the mail type are to be applied on a Project, Company, or Personal basis.
Select one or more Mandatory Recipients to be included in the TO and / or CC distribution for all mails of the selected Type using the company drop down list and right arrow button.
Click OK.

Any user can define Mandatory Recipients on a Personal basis.
Users can only remove mandatory recipients defined by the Project or Company Administrator if the recipient(s) are currently unable to receive the mail type, or the sender is restricted from sending mails to the recipient.