You cannot create mailbox for users with unconfirmed (with an Yet to Verify tag) verified-domain email address. Once the user approves it, the email address can be used for mailbox creation.
Sign in to Zoho Directory, then click Admin Panel in the left menu.
Go to Users, then click on the required user.
Click Create Mailbox. Important: The Create Mailbox button is only available to users
who are onboarded to the organization before a mailbox is configured. If
your organization already has a mailbox set up, newly added users will
not see this option.
Enter an email address for the user. If you have enabled mail hosting in multiple domains, choose the domain in which the mailbox should be created.