Set up Exim to relay certain incoming emails to a different server

Question

I have an exim server, running 4.69.

We are aware of the vulnerability, and due in part we are in the process of migrating to 4.75.

We have an "experimental" 4.75 email server set up, we want to set up our existing server to (preferably sometime in the ACL) check the email for certain conditions and if one is met, we want to relay the email to our experimental server for delivery.

How can I do this? Maybe I need to set up a specific transport for this purpose?

Thank you!

Answer 1

You can to do this with a router. Place the router ahead of any that do local delivery. If necessary, you can configure the router to deliver and pass the message to the normal routers using the unseen optio. See the Exim Specification for more details. Chapter 15 (Generic options for routers is a good place to start configuring the router.

Try this router from Exim FAQ Q0320 replacing the domains line with your criteria.

forward:
  driver = redirect
  domains = some.domain
  data = ${quote:$local_part}@other.domain

Or try this router from Exim FAQ Q0305 replacing the domains line with your criteria.

local_network:
  driver = dnslookup
  transport = remote_smtp
  domains = *.mydomain.com