I can’t get this working either…
Apart from that, there is now a “better” way to log the subject.
Instead of using an “action” of WARN
(which seems too “strong” just to log an item useful for debugging), you can now (as of Postfix 2.8) use the INFO
action instead, which logs an info:
record.
(The fact that you have to use such work-arounds just to log the subject seems weird, but that discussion would be off-topic here…)