Currently, the Jira Service Desk Email Processor Plugin checks the following items in a email header to determine whether to create or comment an issue:

• Subject header - trying to find the issue key and add a comment there
• In-Reply-To header - trying to match the Message-ID value (with the ones in the notificationinstance table) in order to find the issue ID to add a comment to
• References header - same as the previous one
• Message-ID header - same as the previous one

Admins should have the ability to adjust this by the project in case they want issues to always be created regardless of the old reference.

A scenario where this would be useful is: There is an old closed case and we need to create a new one by forwarding the email history. If the email has an old issue key, Jira will comment the old case instead of creating a new one.