It would be really helpful to be able to populate custom mail headers in the email notifications.
We'd like to populate two custom headers, say, X-JIRA-PROJECT and X-JIRA-ISSUE with the project and the issue number. Then our mail clients could sort and group by those two header fields. Possibly we'd also want to add priority or custom fields into these headers.
Another option would be to fill in the In-Response-To header so that messages thread properly, but I'm not sure if that would work as well.