Set the email addresses that receive notifications when your Apex code encounters unhandled exceptions. Emails can be sent to your Salesforce org’s users and to external email addresses.

Salesforce sends two types of notification mails when potential issues with APEX code are detected:

APEX warning mails, which are sent when a class or trigger exceeds 50% of an APEX governor limit
APEX exception mails, which are sent when an unhandled exception occurs in a trigger or class

1. Go to Email –> Apex Exception Email.

2. Click Add Salesforce user to add user to start receiving exception or add External Address in Email Addresses Not in Salesforce section.