[Users] "Thread using subject in addition to standard headers" not working?

[Paul]

On Fri, 8 Jan 2021 15:46:22 +0100
Thomas Leitner <t_leitner at gmx.at> wrote: 

> Here are the headers from a sample ServiceDesk e-mail:
> 
> In-Reply-To: <JIRA.425202.1610015463000 at Atlassian.JIRA>
> References: <JIRA.425202.1610015463000 at Atlassian.JIRA>
> <JIRA.425202.1610015463495 at uniwien-jirasd-1.va.smhss.de>
> 
> All e-mails from the same issue have these same In-Reply-To and
> References headers. The subject is also always the same and thus should
> be usable for threading. But it doesn't work.
> 
> Am I missing something here in how the option works?

Is the parent message in your folder? Threading needs the parent.

with regards

Paul