A valid destination address is determined. An availability request is sent to each destination address from a set of destination addresses. The destination addresses are correlated with a destination party. At least one response to the sent availability requests is received. Each received response is uniquely associated with its own destination address from the destination addresses. Each received response indicates either a valid destination address or an invalid destination address. For each received response, a value associated with the destination address that is associated with that received response is recorded. The value indicates either a valid destination address or an invalid destination address based on the received response associated with that destination address.