[exim] authentication by exim client. multiple smarthost

Hello.

I have 2 smarthosts. Each of these smarthosts requires smtp authentication
for relaying.

I have logins and passwords for both of the smarthosts. login1 - pass1 and
login2 - pass2, for example.

The choice which smarthost should process mail depends on From: header of a
mail message. It's done by routers.

Of course, i cannot use login2-pass2 on the second smarthost (it gives me
authentication error). So I need to
use the specified credentials for specified smarthost.

How can I configure this?

In exim.conf I can add something like:

plain1:
driver = plaintext
public_name = PLAIN
client_send = ^user1^pass1
plain2:
driver = plaintext
public_name = PLAIN
client_send = ^user2^pass2

But it would not solve my problem, because i don't have any possibility to
force exim using user1 when he talks to the first smarthost and user2 - to
the second one.


Thank you.

P.S. it would be very cool to have a possibility to configure the name of
the authenticator(s) which the transport should use.