DNS resolution is genuninely reusable, though. Perhaps that's the test: is this something that could concievably be used by others, as a product in itself, or is it tied very heavily to the business and the rest of the "microservices"?
Remember this is how AWS was born, as a set of "microservices" which could start being sold to external customers, like "storage".
qmail probably counts as "microservices" as well, yes. In that case, for security separation of authority in a multi-user environment. Nowadays we don't really do multi-user environments and separation would be by container.
Remember this is how AWS was born, as a set of "microservices" which could start being sold to external customers, like "storage".