[exim] authenticator options

Hello, I have a fairly basic (I think) question that I haven't been able to
answer via the docs/faqs:

Is it possible for an exim 'authenticator' to use more information than just
the SMTP host for making a choice of which credentials to send? What other
variables are available in the context of an authenticator?

To clarify my question, my existing scenario is that I currently have
messages routed to one of several SMTP servers based on the message's
purpose. All order confirmations are sent via one outgoing server,
newsletters via another, corporate email by another. This is for purposes
of reputation management. I am attempting to transition to a 3rd party mail
sender which provides this functionality (multiple dedicated outgoing IP
addresses -- it's sendgrid.com in case anyone cares), but the way their
system works is that they only have one SMTP server, and the username you
provide in SMTP AUTH is what selects the outgoing IP address they push your
mail through.

So my goal is to have exim look up the credentials to be passed to the SMTP
server on a per-message basis. Ideally, it could be done in a way that
still allows multiple messages which are of the same "type" of email to be
sent on the same connection, but I don't think that is critical.

Basically, I need to be able to make a choice at the transport level about
the authentication parameters in the authenticator. That means either being
able to set a piece of information that the authenticator can read at
authentication time, or having the transport be allowed to explicitly choose
a specific authenticator. Are either of these possible?

-Neal