In addition to predefined review groups, Knak allows you to send an email to an approver 'on the fly' for those situations where no pre-set review group exists.
From the email creation screen select publish > send for review
Select 'Add approvers' from the popup
Set up your review workflow, add the stages you need and populate with reviewers.
You have the option to only need one reviewer to approve the email at any stage. Uncheck 'Everyone in stage must approve' and this stage will only need one reviewer to approve it.
Hit send to activate the approval process.
Need more help? Contact support via live chat within Knak using the chat bubble in the bottom right corner or email firstname.lastname@example.org