Customizing email content
Jira generates emails in reaction to events using a templating engine — Apache’s Velocity. This is a relatively easy to use templating language that can pull apart java objects in useful ways. Whether it’s adding some info to the header or footer, or shuffling around the contents of your emails, you can make a number of customizations and tailor the emails to your needs.
Customizations to Velocity templates or other Jira files are not included in the scope of Atlassian Support.
Which email notifications are you using?
There are two types of notifications in Jira and they're based on different templates. Check which type you're using before you edit the templates.
Default from Jira 8.4
Issue updates that occur close together are grouped and sent in a single email.
Default up to Jira 8.4
Every issue update is sent in a separate email.