I've created a patch that allows to not only create new tickets by subaddressing (like project_from_subaddress) but also to append comments to a ticket by using the ticket id as subaddress. The patch applies to Redmine 3.4.

This allows to forward related mails to 'foobar+1234@example.org' without having to change the mail's subject or message id.

Additionally, I added a new configuration option that let Redmine add a 'Reply-To: foobar+1234@example.org' header field, if enabled. Of course, the server need to have support for subaddressing, otherwise those mails would be rejected. Maybe this could be clarified with an additional description.

The patch doesn't include test cases since I currently don't have the time to setup a complete build environment but changed the code directly in the live system.