That still suggests a _machine race_ could have sent it.

We could define intelligent life as being any of a sapient species, regardless of physical composition. That's helps do away with only carbon-based lifeforms.

What would be the definition of sapience for a machine civilization?

Same as for humans. Or a hive mind, or some other type.

Which is?

Common dictionaries, such as Oxford, state:

Sapient 1. Wise, or attempting to appear wise. 2. Relating to the human species.

Where

1. wise is defined: Having or showing experience, knowledge, and good judgement. Hard to see how that would apply to a hypothetical hive or machine drone civilization with only a single ‘wise’ controller.

2. Clearly not applicable.

Why wouldn't 1 work? Basically it's a species that is at least as intelligent as us and can carry out civilization, including scientific and technological progress. It may or may not be sentient as well, see Blindsight for example.

Sentience might be a better word.

Perhaps "naturally occurring" would be more clear to the layman. Until proven otherwise.