Read and classify an address verified
The address group: read the address as text, ask whether the engine considers it local, and change that answer.
In detail
Three functions on one opaque structure. MQ_local is the interesting one: it exposes the engine's decision about whether an address belongs to this installation, and MQ_setLocal lets a plugin overrule it. That is a routing decision made in C, before anything else sees the message — powerful, and the reason a plugin on a shared node is worth knowing about.