I am moving servers, both of them running Debian (oldstable and stable). Both are running Exim4. When I switch the DNS entries over to the new server, I'd like the old server to still forward incoming mail to my new server for processing, so that users do not see any outage. How can I do this?
How to configure Exim to forward all incoming mail to new server?
email;smtp;exim
Just replace all routers by one manualroute that points directly to the new MX:begin routersredir:  driver = manualroute  domains = *  transport = remote_smtp  route_list = * 12.34.56.78Here 12.34.56.78 - is an IP-address of your new MX where all messages should go.