Adding a comment in existing JIRA issue coming from multiple users via email

[Dean B]

Scenario

Customer 1 raises multiple issues on behalf of 10 different customers (Customers 2 to 11) in JIRA.  Customer 1 is always set as a 'Reporter'.

Once the ticket is raised, the issue reference number (i.e. Key) is sent back to Customer 1. Customer 1 then forwards the relevant issue reference email to relevant customer (2 to 10) to let them know the issue has been raised and the reference number.

Problem

Now Customer 2 wants to know the progress so decides to reply the email which has the project email address and the Issue summary in the subject line. When the response hits JIRA instead of adding back to the existing issue. It raises a new issue.

How do I make sure the response is added in the comments regardless of the sender as long as the subject includes the Issue key and summary?