[otrs] Lock ticket to agent when he answers by email to the OTRS system

Hi ! I made a filter rule from the PostMaster filter module, like this : Subject match: "^LOCK.*" set X-OTRS-Lock to "lock". I'm not sure it's the right way to use X-OTRS-Lock header :S I see in OTRS's log that the rule is well matched, but it doesn't act. I tried with "unlock", it doesn't work either (on a locked ticket, for course) .... How would you do that ? Thank you !! -- Martin