The email notification configuration is located in the /Admin console | Mail Agent | Email Config
By default the "Use MX Lookup?" field is off by default.
Also the "Domain Name" field needs to be populated. For further details please review the following
/admin; then navigate to "Mail Agent | Email Config"; then under the "Email Type Dropdown" select "Request Response". You can see the email that will be sent when the request is approved/denied. The checkbox for "Requestor" needs to be set - but it is set by default. If not then maybe you disabled it at some point