[exim] Authenticate against another server

I'm building a mail submission host, TLS & auth on port 587. I would like
to authenticate against another SMTP server ... Specifically, smtp.gmail.com.
Is there any way to simply pass-through the submitted credentials to
authenticate on another smtp server, for authentication only? I don't want
the other SMTP server to send the mail; I want to use it for authentication
only.



Here's the issue:



We use google apps. But google apps does some email rewriting before the
message goes out from their network. We're building this machine
specifically to use as an alternative to smtp.gmail.com, so email won't be
rewritten while it's being sent. I have to implement some sort of
authentication mechanism. Ideally, I'd like to authenticate using the same
credentials they're currently using. Hence, I want to simply pass-through
the submitted credentials for authentication against another smtp server,
TLS, port 587.



Thanks...