[exim] how to get the email address of the sender?

Hi,

I'm trying to set up a gmail relay:

this works:

begin authenticators

gmail_login:
    driver = plaintext
    public_name = LOGIN
    client_send = : myname@??? : paspaspas

but that is hard coded to be one user, I like the email address part to be
a variable:

begin authenticators

gmail_login:
    driver = plaintext
    public_name = LOGIN
    client_send = : {$sender_address} : paspaspas

this does not work, any idea? Thanks,