[exim] sender-address and gmail

Hi,

I sent a test email as follow:

echo "This is a test." | mail -s Testing -r user1@??? user2@???

then I have this auth for gmail replay:

gmail_login:
    driver = plaintext
    public_name = LOGIN
    client_send = : user1@??? : aPassword

this works, then to make it more generic:

gmail_login:
    driver = plaintext
    public_name = LOGIN
    client_send = : $sender-address : aPassword

this failed, looks like $sender-address is expanded into user1@localhost,
any idea how to make this work? THanks,