[exim] Execute command as part of a Look-up

I am attempting to do the following (simplified example)

domainlist local_domains = @ : ${run{echo "test.com"}}

The above is just to validate you can execute commands as part of a lookup.
Final solution would be to call a script that retrieves the domain list
from an API service.

However the above does not seem to work (obviously doing something wrong or
stupid :) )

Validating an address I see

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Considering mc@???
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
routing mc@???
--------> dnslookup router <--------
local_part=mc domain=test.com
checking domains
direct command:
argv[0] = echo
argv[1] = test.com
test.com in "@ : "? no (end of list)
test.com in "! +local_domains"? yes (end of list)

As you can see test.com is not in the domain list. Although it seems to
indicate that a direct command was called.

Do you have to capture the output a certain way?

Thanks