[exim] @mx_any trouble with match_domain

I am confused. One debugging run contains both...

cvs.infradead.org in "@mx_any"? no (end of list)
    ... and ...
lowest numbered MX record points to local host: cvs.infradead.org

Now surely it can't be both _not_ in the MX list and also the lowest
number MX in the list?

Test case...

imladris /home/dwmw2 # cat exim-mx.conf
begin routers

mx:
driver = redirect
.ifdef USE_DOMAINS
domains = @mx_any
.else
condition = ${if match_domain{$domain}{@mx_any}{yes}}
.endif
data = :fail:Yes we are an MX host for $domain
allow_fail

notmx:
driver = redirect
data = :fail:Not MX
allow_fail

imladris /home/dwmw2 # exim -C exim-mx.conf -bt someuser@???                                                                                
someuser@??? is undeliverable:
  Not MX
imladris /home/dwmw2 # exim -DUSE_DOMAINS -C exim-mx.conf -bt someuser@???
someuser@??? is undeliverable:
  Yes we are an MX host for baythorne.infradead.org

The latter is correct -- the test box _is_ in the MX list for the domain
in question. And it's not just IPv6-related; I've seen this on legacy
IPv4 boxen too.

--
dwmw2