In ITSM, "GetApproval" will generate an Approval, and an Approval Vote tracking. For each Approval Vote, the voter will receive an Email. They can reply to that Email, and include "Approved" or "Denied" in the subject line, to automatically approve or deny their vote...
Open Service Management Administrator > Server Settings > Messaging > Listeners/Invokers. Out of the box, there is an "Email Listener - Approval":