JETI allows you to impose restrictions on recipients of emails in multiple ways in the "Restrictions" section of the General Configuration page.
You can exclude Roles, Groups or Fields from the recipient dropdown fields to avoid accidental broadcast of emails. E.g. you may want to exclude the jira-users group, so that noone is getting tempted to email all users.
By enabling the option "Exclude external recipients" you can make sure that emails will not be sent outside the scope of your JIRA Jira users. Even if an email address is specified manually or stored in a custom field, no emails will be sent to that address unless it belongs to a JIRA Jira user.
Limit to Project User Recipients
By enabling this option, you can make sure that only those JIRA Jira users will receive emails who are authorized to view the issue in JIRAJira. This means who have the "Browse project" permission for the project of the issue.