Example:
Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox')} | Add-MailboxPermission -User tenant_admins@yourdomain.onmicrosoft.com -AccessRights FullAccess -InheritanceType all
Example:
Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox')} | Add-MailboxPermission -User tenant_admins@yourdomain.onmicrosoft.com -AccessRights FullAccess -InheritanceType all