Index: trunk/src/procheader.c
===================================================================
--- trunk/src/procheader.c	(revision 107)
+++ trunk/src/procheader.c	(working copy)
@@ -507,7 +507,7 @@
 	MsgInfo *msginfo;
 	gchar buf[BUFFSIZE], tmp[BUFFSIZE];
 	gchar *reference = NULL;
-	gchar *p;
+	gchar *p, *q;
 	gchar *hp;
 	HeaderEntry *hentry;
 	gint hnum;
@@ -573,6 +573,20 @@
 			msginfo->msgid = g_strdup(hp);
 			break;
 		case H_REFERENCES:
+			p = hp;
+			while ((p = strchr(p, '<')) != NULL && 
+			       (q = strchr(p + 1, '>')) != NULL) {
+				msginfo->references = g_slist_prepend
+					(msginfo->references, 
+					 g_strndup(p + 1, q - p - 1));
+				p = q + 1;						 
+			}
+			if (msginfo->references) { 
+				if (!reference) 
+					/* use last one as possible in-reply-to replacement*/
+					reference = g_strdup(msginfo->references->data);
+			}
+			break;
 		case H_IN_REPLY_TO:
 			if (!reference) {
 				eliminate_parenthesis(hp, '(', ')');
Index: trunk/src/procmsg.c
===================================================================
--- trunk/src/procmsg.c	(revision 107)
+++ trunk/src/procmsg.c	(working copy)
@@ -201,7 +201,9 @@
 	MsgFlags default_flags;
 	gchar file_buf[BUFFSIZE];
 	guint32 num;
+	gint refnum;
 	FolderType type;
+	gchar *ref;
 
 	g_return_val_if_fail(item != NULL, NULL);
 	g_return_val_if_fail(item->folder != NULL, NULL);
@@ -257,6 +259,13 @@
 		READ_CACHE_DATA(msginfo->msgid, fp);
 		READ_CACHE_DATA(msginfo->inreplyto, fp);
 
+		READ_CACHE_DATA_INT(refnum, fp);
+		for (; refnum != 0; refnum--) {
+			READ_CACHE_DATA(ref, fp);
+			msginfo->references = g_slist_append
+				(msginfo->references, ref);
+		}
+
 		MSG_SET_PERM_FLAGS(msginfo->flags, default_flags.perm_flags);
 		MSG_SET_TMP_FLAGS(msginfo->flags, default_flags.tmp_flags);
 
@@ -443,6 +452,7 @@
 void procmsg_write_cache(MsgInfo *msginfo, FILE *fp)
 {
 	MsgTmpFlags flags = msginfo->flags.tmp_flags & MSG_CACHED_FLAG_MASK;
+	GSList *refs;
 
 	WRITE_CACHE_DATA_INT(msginfo->msgnum, fp);
 	WRITE_CACHE_DATA_INT(msginfo->size, fp);
@@ -459,6 +469,11 @@
 	WRITE_CACHE_DATA(msginfo->subject, fp);
 	WRITE_CACHE_DATA(msginfo->msgid, fp);
 	WRITE_CACHE_DATA(msginfo->inreplyto, fp);
+ 
+ 	WRITE_CACHE_DATA_INT(g_slist_length(msginfo->references), fp);
+ 	if (g_slist_length(msginfo->references))
+ 		for (refs = msginfo->references; refs; refs = refs->next) 
+ 			WRITE_CACHE_DATA(((gchar *)refs->data), fp);
 }
 
 void procmsg_write_flags(MsgInfo *msginfo, FILE *fp)
@@ -752,6 +767,7 @@
 	GHashTable *table;
 	MsgInfo *msginfo;
 	const gchar *msgid;
+	GSList *reflist;
 
 	root = g_node_new(NULL);
 	table = g_hash_table_new(g_str_hash, g_str_equal);
@@ -762,7 +778,16 @@
 
 		if (msginfo->inreplyto) {
 			parent = g_hash_table_lookup(table, msginfo->inreplyto);
-			if (parent == NULL)
+			
+			/* check references */			
+			if (!parent && msginfo->references)
+				for (reflist = msginfo->references; 
+				     reflist != NULL; 
+				     reflist = reflist->next)
+					if ((parent = g_hash_table_lookup
+						(table, reflist->data))) 
+						break;
+			if (parent == NULL) 
 				parent = root;
 		}
 		node = g_node_insert_data_before
@@ -777,16 +802,28 @@
 	for (node = root->children; node != NULL; ) {
 		next = node->next;
 		msginfo = (MsgInfo *)node->data;
-		if (msginfo->inreplyto) {
+		parent = NULL;
+
+		/* check in reply to */
+		if (msginfo->inreplyto)
 			parent = g_hash_table_lookup(table, msginfo->inreplyto);
-			/* node should not be the parent, and node should not
-			   be an ancestor of parent (circular reference) */
-			if (parent && parent != node &&
-			    !g_node_is_ancestor(node, parent)) {
-				g_node_unlink(node);
-				g_node_insert_before
-					(parent, parent->children, node);
-			}
+
+		/* check references */			
+		if (!parent && msginfo->references)
+			for (reflist = msginfo->references; 
+			     reflist != NULL; 
+			     reflist = reflist->next)
+				if ((parent = g_hash_table_lookup
+					(table, reflist->data))) 
+					break;
+		
+		/* node should not be the parent, and node should not
+		   be an ancestor of parent (circular reference) */
+		if (parent && parent != node &&
+		    !g_node_is_ancestor(node, parent)) {
+			g_node_unlink(node);
+			g_node_insert_before
+				(parent, parent->children, node);
 		}
 		node = next;
 	}
@@ -1419,10 +1456,19 @@
 	return full_msginfo;
 }
 
+static void free_references(gpointer data, gpointer user)
+{
+	g_free(data);
+}
+
 void procmsg_msginfo_free(MsgInfo *msginfo)
 {
 	if (msginfo == NULL) return;
 
+	g_slist_foreach(msginfo->references, 
+		       free_references, NULL);
+	g_slist_free(msginfo->references);
+
 	g_free(msginfo->xface);
 
 	g_free(msginfo->fromname);
Index: trunk/src/procmsg.h
===================================================================
--- trunk/src/procmsg.h	(revision 107)
+++ trunk/src/procmsg.h	(working copy)
@@ -180,6 +180,7 @@
 	gchar *subject;
 	gchar *msgid;
 	gchar *inreplyto;
+	GSList *references;
 
 	FolderItem *folder;
 	FolderItem *to_folder;
Index: trunk/src/summaryview.c
===================================================================
--- trunk/src/summaryview.c	(revision 107)
+++ trunk/src/summaryview.c	(working copy)
@@ -3033,6 +3033,7 @@
 	GtkCTreeNode *next;
 	GtkCTreeNode *parent;
 	MsgInfo *msginfo;
+	GSList *reflist;
 
 	summary_lock(summaryview);
 
@@ -3053,6 +3054,17 @@
 		if (msginfo && msginfo->inreplyto) {
 			parent = g_hash_table_lookup(summaryview->msgid_table,
 						     msginfo->inreplyto);
+						     
+			/* check references */			
+			if (!parent && msginfo->references)
+				for (reflist = msginfo->references; 
+				     reflist != NULL; 
+				     reflist = reflist->next)
+					if ((parent = g_hash_table_lookup
+						(summaryview->msgid_table, 
+						 reflist->data))) 
+						break;
+						
 			if (parent && parent != node) {
 				gtk_ctree_move(ctree, node, parent, NULL);
 				gtk_ctree_expand(ctree, node);
Index: trunk/src/defs.h
===================================================================
--- trunk/src/defs.h	(revision 107)
+++ trunk/src/defs.h	(working copy)
@@ -63,7 +63,7 @@
 #define FOLDER_LIST		"folderlist.xml"
 #define CACHE_FILE		".sylpheed_cache"
 #define MARK_FILE		".sylpheed_mark"
-#define CACHE_VERSION		0x20
+#define CACHE_VERSION		0x1024
 #define MARK_VERSION		2
 
 #define DEFAULT_SIGNATURE	".signature"
@@ -105,4 +105,7 @@
 
 #define DEFAULT_MESSAGE_FONT	"Sans 14"
 
+#undef CACHE_VERSION
+#define CACHE_VERSION			0x1024
+
 #endif /* __DEFS_H__ */