In each tab within customization, you'll find the option "Mail Comments.
When checked, affected users will be emailed when comments are posted to an item within Testersuite. By affected users, Testersuite means the following:
Test cycles, Products, Requirements, Test Cases, Test Scenarios & defects
For these items in Testersuite , comments will be emailed to all users and/or groups entered in user fields and user and group fields. For example, the user designated as the tester of a Requirement, or the group designated as the handler of a defect.
Testruns
The above rule also applies to emailing comments in Testruns: all users and/or groups entered in user fields and user and group fields are emailed when the setting is on when posting comments. In addition, in the case of Testruns, testers who have one or more test cases to their name within the given Testrun are also emailed. In this way, you as for example test coordinator can easily notify all testers within a Testrun of important changes or other information.