[Exim] Envelope Recipient in user .forward file

Hi,

I am currently receiving e-mail for a virtual domain example.com to my
account and would like to sort e-mail for that domain by the envelope
recipient. As far as I can see, there is no expansion variable for a
message's envelope recipient in the user .forward file, and
$header_envelope-to: seems to be empty even if the incoming message
has a Envelope-to: header line. At least, this applies for messages
piped into exim -bf.

Am I doing something wrong? What is the recommended way to access a
message's envelope recipient from a user .forward file?

Any hints will be appreciated.

Greetings
Marc