If email has been deferred to a member on a local mailing list (implemented through forward files), and one of our ETRN clients is on this mailing list, the -R won't flush the mailing list message for that client.

That is because -R matches only original recipient addresses, not those produced as a result of expansion, because these are not (by default) preserved from delivery to delivery. You can get round this by setting one_time on the forwarding router, but you are not allowed to have expansions to pipes or files on routers that have one_time set. Therefore, you will have to have a separate router for mailing lists (with one_time set) to the one used for normal forward files that might specify pipe or file deliveries. However, the problem will still be present for any user who sets up a .forward file to redirect to any of the ETRN domains. See the last 3 paragraphs of `../Q1407`_ for a discussion of an alternative approach.