How do I target a different mail server depending on domain with exim?

I have an Ubuntu VPS/slice at Slicehost. Right now I use Exim (Exim4) as my mechanism to send email. I don't receive email on my slice; I only send email (for example, if someone leaves a comment on my blog, an email is sent to me telling me so).

I have a /etc/exim4/passwd.client file that has my credential information for sending the email, of the form:

### target.mail.server.example:login:password

Does anyone know a way how I can have multiple target mail servers depending on the website/domain visited? For example, I have one target mail server for when someone leaves a comment on my blog and another when sending out an order confirmation to someone.

Thank you!

The From address isn't controlled by exim. The software submitting the message, blog or ecommerce site, has to set it.

As David said, I'm not sure if Google checks the from header, so you may have to add both from address into google first (settings -> account -> add email)