When replying to an e-mail that was sent to create a JIRA issue, instead of adding a comment to the related issue, as it should according to this documentation, the reply e-mail is creating a new JIRA issue.
As he does not find the JIRA Issue Key in the e-mail subject, he should check the in-reply-to header of the e-mail to see if the first e-mail of the chain created an issue, and it seems the system is not doing that.

Steps to reproduce

1. Send an e-mail to create and issue with a Cc to yourself or another e-mail.
   1. This e-mail has the example ID: 1234
2. Wait for the issue to be created.
   1. JIRA replies with an e-mail that has the example ID: ABCD
3. Now, using the CC address, just reply the email with ID: 1234 that created the issue.

The handler is not following the In-reply-to ID.

Expected behavior - The reply should be added as a comment to the issue.
Actual behavior - The reply is creating a new issue. (Duplicated one)