Option to disable email header reference (In-reply-to) check in JSM mail handler

Issue Summary

To determines if the content of incoming email messages becomes a new issue or a comment on an existing issue there two checks.

1. Jira/JSM mail handler first looks for issuekey in subject line of email. If key is found, then it gets added as comment.
2. If no issuekey is found on the subject line, then mail handler scans message header for "In-reply-to" and "References" fields. From these header fields, the mail handler tries to detect if there is any message-ID that is linked to an existing Jira ticket. If any message-ID is found in the 2 headers above, the mail handler will analyze it and try to map it to an existing Jira ticket as a comment.

See diagram:

Problem:

In an ideal world sender of the email should create a new email when they want to New Issue to be created. However, usually end users repurpose existing email (by clearing the subject line and body) then write new sub/body. In these scenario Jira will add emails as comment to existing issue. 

For organisation serving large number of external customers its impossible to control or educate all how they send email.

Suggestion:

Give an option to admin to disable reference (In-reply-to) check in JSM mail handler and only use issue key in subject line to determine if it should be comment.  

Workaround

Currently there are no workaround available for Cloud.