Added valid but unknown mx user

src/Invite.js

@@ -59,6 +59,12 @@ export function inviteMultipleToRoom(roomId, addrs) {
     return this.inviter.invite(addrs);
 }
 
+/**
+ * Checks is the supplied address is valid
+ *
+ * @param {addr} The mx userId or email address to check
+ * @returns true, false, or null for unsure
+ */
 export function isValidAddress(addr) {
     // Check if the addr is a valid type
     var addrType = this.getAddressType(addr);
@@ -67,7 +73,7 @@ export function isValidAddress(addr) {
         if (user) {
             return true;
         } else {
-            return false;
+            return null;
         }
     } else if (addrType === "email") {
         return true;