[exim] option "envelope_to_remove" unknown

Hello,

I'm using the 4.43 release of exim. As all messages to external
mailservers are sent through our smarthost, this smarthost shall not
change any headers, especially not the Envelope-to:-header.

As of the documentation, I tried to set

envelope_to_remove = false


documentation:
envelope_to_remove
Type: boolean
Default: true

But when I start exim I get an error:

2005-01-20 15:36:45 Exim configuration error in line 396 of
/vrmd/mailout/exim/configure:
option "envelope_to_remove" unknown

Whats that?

Regards
Marten