My initial thought was to make an exim&lowast configuration change to do this, but a simple look through the documentation failed to produce an obvious way of doing this without blocking mail to root locally.
However, when I asked on IRC Diziet (WINOLJ) pointed out that this could be done with SAUCE. Since chiark provides my exterior MX services I already have SAUCE in the equation, but have until recently had all addresses classified as
laxwhich means that SAUCE won't (generally) reject stuff.
So, a quick conversation with mailadmin later to arrange some delegation I could configure SAUCE to issue a 550 error for all mail to root saying "use appropriate role address".
Hopefully this will reduce my spam intake by about 33% for ysolde.ucam.org.
∗ Version 3.35. People wishing to rant about the antiquity of this should direct their effort at getting Debian Sarge released.