[exim] Question about conditions in routers - Exim 3.35

Hello!

I'm trying to implement some filtering of messages into certain routers
according to a certain header field is set or not.
As far i have tried following:
condition = {if def:header_X-crypt: {0}{1}}
The header field is set by some script i'm piping the mails through. As long
as this condition doesn't match, the mails are sent in an endless loop as
every mail gets sent to the script again, which sets the header and returns
the mail to exim.
Perhaps anyone of you could tell me what's wrong here :)
Thanks in advance!

Greetings,
Matthias