"In-Reply-Header" missing if one is not recipient of "issue created" email.

The MailThreadManagerImpl.java contains method threadNotificationEmail that creates threaded mail only for recipients of NOTIFICATION_ISSUE_CREATED.

If we change the line 18 from

notificationGV = EntityUtil.getFirst(genericDelegator.findByAnd("NotificationInstance", EasyMap.build("source", issueId, "type", NOTIFICATION_ISSUE_CREATED, "email", email.getTo()), EasyList.build("id")));

to

notificationGV = EntityUtil.getFirst(genericDelegator.findByAnd("NotificationInstance", EasyMap.build("source", issueId, "email", email.getTo()), EasyList.build("id")));

Then threaded emails will go to all people involved in the issue. It is because the first email to the specified person will be treated into consideration (does not metter if it is issue creaed or other).