[otrs] PostMaster Filter matching undefined values

Hey all, So, I want to set up a mail rule along the lines of Match: "To" being "mail@domain.tld", X-OTRS-Queue not being set Set: X-OTRS-Queue: Queue::Example However, this rule only coming into play when X-OTRS-Queue is not set, is important otherwise these emails land in the wrong folder, or it overwrites the already set X-OTRS-Queue header in some emails. Any help on matching this undefined value? The regular expression support isn't so regular ;) I've attempted: /^$/, ^$, (^$), (/^$/), amongst other things, all to no avail. Also, attempting to submit the form without a value set whilst in the admin panel doesn't work. I'm using OTRS 2.1.4 on a Ubuntu based box, with a MySQL backend. Thanks in advance; -- Muz