Index: Lib/mailbox.py
===================================================================
--- Lib/mailbox.py	(revision 53019)
+++ Lib/mailbox.py	(working copy)
@@ -237,6 +237,8 @@
             else:
                 raise NoSuchMailboxError(self._path)
         self._toc = {}
+        self._cur_mtime = None          # Records mtime of 'cur' directory
+        self._new_mtime = None          # Records mtime of 'cur' directory
 
     def add(self, message):
         """Add message and return assigned key."""
@@ -457,12 +459,24 @@
 
     def _refresh(self):
         """Update table of contents mapping."""
+        new_mtime = os.path.getmtime(os.path.join(self._path, 'new'))
+        cur_mtime = os.path.getmtime(os.path.join(self._path, 'cur'))
+
+        if new_mtime == self._new_mtime and cur_mtime == self._cur_mtime:
+            return
+        
         self._toc = {}
-        for subdir in ('new', 'cur'):
-            for entry in os.listdir(os.path.join(self._path, subdir)):
+        def update_dir (subdir):
+            path = os.path.join(self._path, subdir)
+            for entry in os.listdir(path):
                 uniq = entry.split(self.colon)[0]
                 self._toc[uniq] = os.path.join(subdir, entry)
-
+                
+        update_dir('new')
+        update_dir('cur')
+        self._new_mtime = new_mtime
+        self._cur_mtime = cur_mtime
+                
     def _lookup(self, key):
         """Use TOC to return subpath for given key, or raise a KeyError."""
         try: