Hi,
I'am facing the similar issue that @Gary Rafter had in this question:
https://community.atlassian.com/t5/Jira-Service-Desk-questions/JSD-is-creating-a-new-ticket-for-each-email-response/qaq-p/796436
but I think in my case it has nothing to do with domains.
Our configuration:
1. We have a project in Jira SD dedicated to only one customer.
2. Customer sends emails from their ticketing system to our email box (Gmail). It is always the same sender email.
3. We have configured email handler that monitors our box and create tickets when email from our customer appears. In handler we choose option: Create a new issue or add a comment to an existing issue
4. When our SD Team respond to this ticket, the answer is added to ticket comment and email is send to customer.
5. When customer answers the separate ticket is created. The subject of email is the same.
When I run some test with my private Gmail account everything was correct. Answers was properly added to comments of existing ticket. I'm pretty sure that this is something to do with email headers changed somehow in our customer ticketing system but I'm not sure how I can be sure about that. The only info I can find in "atlassian-jira-incoming-mail.log" is that "Issue XYZ created"
We have Jira Service Desk: 3.13.2
Regards
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Seweryn