Index: Lib/mailbox.py
===================================================================
--- Lib/mailbox.py	(revision 54661)
+++ Lib/mailbox.py	(working copy)
@@ -492,6 +492,25 @@
                 continue
 
 
+class _recordingDict(dict):
+    def __init__(self, *args, **kwargs):
+        super(_recordingDict, self).__init__(*args, **kwargs)
+        self.all_offsets = set(tuple(value) for value in self.itervalues())
+    def __setitem__(self, key, value):
+        self.all_offsets.add(value)
+        super(_recordingDict, self).__setitem__(key, value)
+    def update(self, *args, **kwargs):
+        as_dict = dict(*args, **kwargs)
+        self.all_offsets.update(tuple(value) for value in as_dict.itervalues())
+        super(_recordingDict, self).update(as_dict)
+    def setdefault(self, key, default=None):
+        try:
+            return self[key]
+        except KeyError:
+            self[key] = default
+            return default
+
+
 class _singlefileMailbox(Mailbox):
     """A single-file mailbox."""
 
@@ -562,6 +581,7 @@
                 warnings.warn("lock() method called with pending changes; "
                               "should have been locked before making changes",
                               stacklevel=2)
+            self._update_toc()
 
     def unlock(self):
         """Unlock the mailbox if it is locked."""
@@ -599,7 +619,7 @@
         remove_temp_file = True
         new_file = _create_temporary(self._path)
         try:
-            new_toc = {}
+            new_toc = _recordingDict()
             self._pre_mailbox_hook(new_file)
             for key in sorted(self._toc.keys()):
                 start, stop = self._toc[key]
@@ -653,6 +673,7 @@
                     orig_file.close()
                     if self._locked:
                         _lock_file(self._file, dotlock=False)
+                        self._update_toc()
 
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""
@@ -676,13 +697,42 @@
     def _lookup(self, key=None):
         """Return (start, stop) or raise KeyError."""
         if self._toc is None:
-            self._generate_toc()
+            self._update_toc()
         if key is not None:
             try:
                 return self._toc[key]
             except KeyError:
                 raise KeyError('No message with key: %s' % key)
 
+    def _update_toc(self):
+        """Reread file, and merge new messages into self._toc."""
+        old_toc = self._toc
+        our_next_key = self._next_key
+        try:
+            self._toc = None
+            self._next_key = 0
+            self._generate_toc()
+            new_toc = self._toc
+        finally:
+            self._toc = old_toc
+            self._next_key = our_next_key
+        new_offsets = frozenset(tuple(value) for value in new_toc.itervalues())
+        if self._toc is None:
+            self._toc = _recordingDict()
+        if self._pending:
+            old_offsets = self._toc.all_offsets
+        else:
+            old_offsets = frozenset(tuple(value) for value in
+                                    self._toc.itervalues())
+        if not old_offsets.issubset(new_offsets):
+            raise ExternalClashError('Existing messages have been modified, '
+                                     'moved or deleted.')
+        for offsets in sorted(new_offsets - old_offsets):
+            self._toc[self._next_key] = offsets
+            self._next_key += 1
+        self._file.seek(0, 2)
+        self._file_length = self._file.tell()
+
     def _append_message(self, message):
         """Append message to mailbox and return (start, stop) offsets."""
         self._file.seek(0, 2)
@@ -782,7 +832,6 @@
                 break
         self._toc = dict(enumerate(zip(starts, stops)))
         self._next_key = len(self._toc)
-        self._file_length = self._file.tell()
 
 
 class MMDF(_mboxMMDF):
@@ -826,8 +875,6 @@
                 break
         self._toc = dict(enumerate(zip(starts, stops)))
         self._next_key = len(self._toc)
-        self._file.seek(0, 2)
-        self._file_length = self._file.tell()
 
 
 class MH(Mailbox):
@@ -1238,8 +1285,6 @@
         self._toc = dict(enumerate(zip(starts, stops)))
         self._labels = dict(enumerate(label_lists))
         self._next_key = len(self._toc)
-        self._file.seek(0, 2)
-        self._file_length = self._file.tell()
 
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""