If you provide a valid address but from a different block-chain (I guess) for example 1Cm554iqhU9dLnZsi2LA5s1b576DdfM1GMwhen working ontestnet` the bitcoind returns the following object:

address: "msH2N7opWVat7u3VRbJXunDuw6gvdoTbc2" ismine: false isvalid: true

Completely unrelated to my original request.

Haven't tested it thoroughly yet, will update when I have more data.