In order to allow Approve/Reject via email you need:
The email's subject follows the convention below: Outgoing Email prefix - Project name / (Issue Key) Issue Summary / Source Workflow Status - Approval Required for 'Workflow Transition'.
The approver can APPROVE/REJECT just replying to the received email by adding the following tags:
Moreover what you add after the previous tag is considered as an approval/rejection comment.
It's required that you always apply the tag @ to end your reply.