Incoming mail handler creates new issue instead of a comment when the subject line is the same

[Leonid_Zadonaisky]

Hey,

I'm running JIRA server 7.13.3 and I am trying to figure out the issue I'm having with the incoming emails. 

My configurations are:

• Direct connection mail handler to JIRA 
• Mail handler is configured to create or comment 

Test Scenario:

When I am sending an email that is being picked up by the mail handler it is opening as case as expected, for example:
from: leon@test.com
to: jira-incoming@test.com
cc: leon@test.com

Then when I reply on the mail (the mail that comes from the cc, not from JIRA) it adds a comment as expected.

Then, If I'll send a new email with the same subject line it will open up a new case instead of comment on the exciting one.

Real Scenario:

A user sends an email when:
to: jira-incoming@test.com
cc: a bunch of people that have also jira users

Each time that some person (from the cc list) reply to the email, a new case is opened.

Would appreciate your help