In case you want to render issue link information in your email, you must edit your email templates. Rendering issue links requires a complex processing logic. The key is $!issue.linkCollection that returns a LinkCollection object if there are linked issues ortherwise null.
Below are example template code fragments that will render issue link information in HTML or TEXT emails.
When selecting "Linked Issues" in the Field Picker dropdown, you will get instructions on how to easily include issue link information in the template.
In case you want a customized display (e.g. include more attributes of the linked issues), then you can start to add these template fragments directly into your templates and customize them according to your needs.
JIRA Security Applied
The LinkCollection will return the linked issues as the email sender sees them. The sender may not be privileged to view all linked issues in JIRA. If this is the case, the email may not list all linked issues either.