--- core/src/main/java/quickfix/DataDictionary.java (original)
+++ core/src/main/java/quickfix/DataDictionary.java
@@ -617,9 +617,11 @@
             }
 
             if (beginString != null && shouldCheckTag(field)) {
-                dd.checkValidTagNumber(field);
-                if (map instanceof Message) {
-                    checkIsInMessage(field, msgType);
+                if(!allowUnknownMessageFields) {
+                    dd.checkValidTagNumber(field);
+                    if (map instanceof Message) {
+                        checkIsInMessage(field, msgType);
+                    }
                 }
                 dd.checkGroupCount(field, map, msgType);
             }
@@ -741,7 +743,7 @@
 
     // / Check if a field is in this message type.
     private void checkIsInMessage(Field<?> field, String msgType) {
-        if (!isMsgField(msgType, field.getField()) && !allowUnknownMessageFields) {
+        if (!isMsgField(msgType, field.getField())) {
             throw new FieldException(SessionRejectReason.TAG_NOT_DEFINED_FOR_THIS_MESSAGE_TYPE,
                     field.getField());
         }