Email approval response and approval request notifications
Key Takeaway
With email approval response on, an approver with the API Enabled permission can approve or reject by putting a supported word on the first line of the reply. Email approval response isn't supported for Classic Approval Processes that assign the approval to a queue.
When an approval process assigns a request to someone, Salesforce emails that approver to say a request is waiting. If an admin turns on email approval response, the approver or a delegated approver can approve or reject by replying to that email, without opening Salesforce.
The approval request email
When a Classic Approval Process assigns an approval request to a user, Salesforce sends that user an approval request email. If Approvals in Chatter is enabled, Salesforce also posts the request to Chatter.
If you assign the same user to an approval request more than once in a single step, Salesforce sends that user only one request.