> Am I right that, in both cases, if the router run, it can not decline?
Yes for (1), no for (2).
The dnslookup router will decline if the domain does not exist in DNS,
or the domain is not syntactically valid for SMTP's defined DNS lookups,
or the MX record points to ".", which is a convention for "no MX service
for this domain, and do not fall back to A/AAAA lookups". There's a
couple of other cases too.
Just because the domain is listed in +relay_to_domains, that doesn't
mean that +relay_to_domains is up-to-date, or that the recipient's DNS
is not just broken.
> I think spec.txt mentions a similar situation in a discussion about
> the manualroute.
This is where manualroute is so very different from dnslookup. For
manualroute, each domain is handled by explicit configuration, whereas
dnslookup involves going out into the wilds of public DNS, to see
whether or not the domain might exist and discover what policy might
exist for where to send mail for that domain.
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