Allowing customers to close a ticket via email?

[JeroenB]

I would like my customers to be able to close existing tickets via an email containing for example something along the line of "case closed".

So I created a PostMaster filter, looking at the body of the email for "case closed*", whereupon the X-OTRS-State will be changed in "closed succesfull":

When I try and send an email with only the string "case closed" the resulting ticket gets closed automatically, and that's great Except this will never happen in real situations, when I create a ticket first and then respond to an Agent email from that ticket with the string "case closed" the ticket does not get closed: is it possible to get this working?

Thanks in advance!