[Exim] different smarthosts depending on SENDER domain

Hi, I would normally want my localhost smtp server running exim, to
send all messages directly to their destination. However, since I am on
a dialup account, many destination smtp servers refuse to accept mail
from my address as an over zealous anti-spam measure.

I solved this for myself, my normal email address, @gmx.de, by using the
smtp server at gmx.net as a smarthost, but of course all my other users
can't use this, some are using our dynamic dns address as their email
domain, and some use email addresses provided by our isp, t-online.de.

How can I write exim rules, that specify mail.gmx.net as the smarthost
ONLY when the From address is gmx.de, and specify smtp.t-online.com,
only when the senders address is t-online.de, and for the local exim to
deliver directly for all other cases?

Incidentally, these email addresses, are translated from the local user
codes into their real addresses in the /etc/email-addresses file, part
of exim.

Thanks for your assistance.

Kurt Häusler

PS, You can look at the header of this email, to see the path emails are
currently following, if that helps.