diff --git a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
index eab2710c2..96012c3e6 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java
@@ -41,6 +41,7 @@ import java.util.logging.Level;
 import java.util.logging.Logger;
 
 import org.jivesoftware.smack.ConnectionConfiguration.SecurityMode;
+import org.jivesoftware.smack.SmackConfiguration.UnknownIqRequestReplyMode;
 import org.jivesoftware.smack.SmackException.AlreadyConnectedException;
 import org.jivesoftware.smack.SmackException.AlreadyLoggedInException;
 import org.jivesoftware.smack.SmackException.NoResponseException;
@@ -991,20 +992,31 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
         replyTimeout = timeout;
     }
 
-    private static boolean replyToUnknownIqDefault = true;
-
     /**
      * Set the default value used to determine if new connection will reply to unknown IQ requests. The pre-configured
      * default is 'true'.
      *
      * @param replyToUnkownIqDefault
      * @see #setReplyToUnknownIq(boolean)
+     * @deprecated Use {@link SmackConfiguration#setUnknownIqRequestReplyMode(org.jivesoftware.smack.SmackConfiguration.UnknownIqRequestReplyMode)} instead.
      */
+    @Deprecated
+    // TODO Remove in Smack 4.3
     public static void setReplyToUnknownIqDefault(boolean replyToUnkownIqDefault) {
-        AbstractXMPPConnection.replyToUnknownIqDefault = replyToUnkownIqDefault;
+        SmackConfiguration.UnknownIqRequestReplyMode mode;
+        if (replyToUnkownIqDefault) {
+            mode = SmackConfiguration.UnknownIqRequestReplyMode.replyServiceUnavailable;
+        } else {
+            mode = SmackConfiguration.UnknownIqRequestReplyMode.doNotReply;
+        }
+        SmackConfiguration.setUnknownIqRequestReplyMode(mode);
     }
 
-    private boolean replyToUnkownIq = replyToUnknownIqDefault;
+    private SmackConfiguration.UnknownIqRequestReplyMode unknownIqRequestReplyMode = SmackConfiguration.getUnknownIqRequestReplyMode();
+
+    public void setUnknownIqRequestReplyMode(UnknownIqRequestReplyMode unknownIqRequestReplyMode) {
+        this.unknownIqRequestReplyMode = Objects.requireNonNull(unknownIqRequestReplyMode, "Mode must not be null");
+    }
 
     /**
      * Set if Smack will automatically send
@@ -1012,9 +1024,18 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
      * registered {@link IQRequestHandler} is received.
      *
      * @param replyToUnknownIq
+     * @deprecated use {@link #setUnknownIqRequestReplyMode(UnknownIqRequestReplyMode)} instead.
      */
+    @Deprecated
+    // TODO Remove in Smack 4.3
     public void setReplyToUnknownIq(boolean replyToUnknownIq) {
-        this.replyToUnkownIq = replyToUnknownIq;
+        SmackConfiguration.UnknownIqRequestReplyMode mode;
+        if (replyToUnknownIq) {
+            mode = SmackConfiguration.UnknownIqRequestReplyMode.replyServiceUnavailable;
+        } else {
+            mode = SmackConfiguration.UnknownIqRequestReplyMode.doNotReply;
+        }
+        unknownIqRequestReplyMode = mode;
     }
 
     protected void parseAndProcessStanza(XmlPullParser parser) throws Exception {
@@ -1089,13 +1110,24 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
                     throw new IllegalStateException("Should only encounter IQ type 'get' or 'set'");
                 }
                 if (iqRequestHandler == null) {
-                    if (!replyToUnkownIq) {
+                    XMPPError.Condition replyCondition;
+                    switch (unknownIqRequestReplyMode) {
+                    case doNotReply:
                         return;
+                    case replyFeatureNotImplemented:
+                        replyCondition = XMPPError.Condition.feature_not_implemented;
+                        break;
+                    case replyServiceUnavailable:
+                        replyCondition = XMPPError.Condition.service_unavailable;
+                        break;
+                    default:
+                        throw new AssertionError();
                     }
+
                     // If the IQ stanza is of type "get" or "set" with no registered IQ request handler, then answer an
                     // IQ of type 'error' with condition 'service-unavailable'.
                     ErrorIQ errorIQ = IQ.createErrorResponse(iq, XMPPError.getBuilder((
-                                    XMPPError.Condition.service_unavailable)));
+                                    replyCondition)));
                     try {
                         sendStanza(errorIQ);
                     }
diff --git a/smack-core/src/main/java/org/jivesoftware/smack/SmackConfiguration.java b/smack-core/src/main/java/org/jivesoftware/smack/SmackConfiguration.java
index 2e6770c7a..dafd97810 100644
--- a/smack-core/src/main/java/org/jivesoftware/smack/SmackConfiguration.java
+++ b/smack-core/src/main/java/org/jivesoftware/smack/SmackConfiguration.java
@@ -34,6 +34,7 @@ import org.jivesoftware.smack.debugger.SmackDebugger;
 import org.jivesoftware.smack.debugger.SmackDebuggerFactory;
 import org.jivesoftware.smack.parsing.ExceptionThrowingCallback;
 import org.jivesoftware.smack.parsing.ParsingExceptionCallback;
+import org.jivesoftware.smack.util.Objects;
 
 /**
  * Represents the configuration of Smack. The configuration is used for:
@@ -370,4 +371,20 @@ public final class SmackConfiguration {
         return defaultHostnameVerififer;
     }
 
+    enum UnknownIqRequestReplyMode {
+        doNotReply,
+        replyFeatureNotImplemented,
+        replyServiceUnavailable,
+    }
+
+    // TODO Change to replyFeatureNotImplemented in Smack 4.3
+    private static UnknownIqRequestReplyMode unknownIqRequestReplyMode = UnknownIqRequestReplyMode.replyServiceUnavailable;
+
+    public static UnknownIqRequestReplyMode getUnknownIqRequestReplyMode() {
+        return unknownIqRequestReplyMode;
+    }
+
+    public static void setUnknownIqRequestReplyMode(UnknownIqRequestReplyMode unknownIqRequestReplyMode) {
+        SmackConfiguration.unknownIqRequestReplyMode = Objects.requireNonNull(unknownIqRequestReplyMode, "Must set mode");
+    }
 }