diff -r 2b79b4848f44 Lib/mailbox.py
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -208,6 +208,9 @@ class Mailbox:
             raise ValueError("String input must be ASCII-only; "
                 "use bytes or a Message instead")
 
+    # Whether each message must end in a newline
+    _append_newline = False
+
     def _dump_message(self, message, target, mangle_from_=False):
         # This assumes the target file is open in binary mode.
         """Dump message contents to target file."""
@@ -219,6 +222,9 @@ class Mailbox:
             data = buffer.read()
             data = data.replace(b'\n', linesep)
             target.write(data)
+            if self._append_newline and not data.endswith(linesep):
+                # Make sure the message ends with a newline
+                target.write(linesep)
         elif isinstance(message, (str, bytes, io.StringIO)):
             if isinstance(message, io.StringIO):
                 warnings.warn("Use of StringIO input is deprecated, "
@@ -230,11 +236,15 @@ class Mailbox:
                 message = message.replace(b'\nFrom ', b'\n>From ')
             message = message.replace(b'\n', linesep)
             target.write(message)
+            if self._append_newline and not message.endswith(linesep):
+                # Make sure the message ends with a newline
+                target.write(linesep)
         elif hasattr(message, 'read'):
             if hasattr(message, 'buffer'):
                 warnings.warn("Use of text mode files is deprecated, "
                     "use a binary mode file instead", DeprecationWarning, 3)
                 message = message.buffer
+            lastline = None
             while True:
                 line = message.readline()
                 # Universal newline support.
@@ -248,6 +258,10 @@ class Mailbox:
                     line = b'>From ' + line[5:]
                 line = line.replace(b'\n', linesep)
                 target.write(line)
+                lastline = line
+            if self._append_newline and lastline and not lastline.endswith(linesep):
+                # Make sure the message ends with a newline
+                target.write(linesep)
         else:
             raise TypeError('Invalid message type: %s' % type(message))
 
@@ -833,30 +847,48 @@ class mbox(_mboxMMDF):
 
     _mangle_from_ = True
 
+    # All messages must end in a newline character, and
+    # _post_message_hooks outputs an empty line between messages.
+    _append_newline = True
+
     def __init__(self, path, factory=None, create=True):
         """Initialize an mbox mailbox."""
         self._message_factory = mboxMessage
         _mboxMMDF.__init__(self, path, factory, create)
 
-    def _pre_message_hook(self, f):
-        """Called before writing each message to file f."""
-        if f.tell() != 0:
-            f.write(linesep)
+    def _post_message_hook(self, f):
+        """Called after writing each message to file f."""
+        f.write(linesep)
 
     def _generate_toc(self):
         """Generate key-to-(start, stop) table of contents."""
         starts, stops = [], []
+        last_was_empty = False
         self._file.seek(0)
         while True:
             line_pos = self._file.tell()
             line = self._file.readline()
             if line.startswith(b'From '):
                 if len(stops) < len(starts):
-                    stops.append(line_pos - len(linesep))
+                    if last_was_empty:
+                        stops.append(line_pos - len(linesep))
+                    else:
+                        # The last line before the "From " line wasn't
+                        # blank, but we consider it a start of a
+                        # message anyway.
+                        stops.append(line_pos)
                 starts.append(line_pos)
+                last_was_empty = False
             elif not line:
-                stops.append(line_pos)
+                if last_was_empty:
+                    stops.append(line_pos - len(linesep))
+                else:
+                    stops.append(line_pos)
                 break
+            elif line == linesep:
+                last_was_empty = True
+            else:
+                last_was_empty = False
         self._toc = dict(enumerate(zip(starts, stops)))
         self._next_key = len(self._toc)
         self._file_length = self._file.tell()
diff -r 2b79b4848f44 Lib/test/test_mailbox.py
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -1116,6 +1116,29 @@ class TestMbox(_TestMboxMMDF, unittest.TestCase):
             perms = st.st_mode
             self.assertFalse((perms & 0o111)) # Execute bits should all be off.
 
+    def test_terminating_newline(self):
+        message = email.message.Message()
+        message['From'] = 'john@example.com'
+        message.set_payload('No newline at the end')
+        i = self._box.add(message)
+
+        # A newline should have been appended to the payload
+        message = self._box.get(i)
+        self.assertEqual(message.get_payload(), 'No newline at the end\n')
+
+    def test_message_separator(self):
+        # Check there's always a single blank line after each message
+        self._box.add('From: foo\n\n0')  # No newline at the end
+        with open(self._path) as f:
+            data = f.read()
+            self.assertEqual(data[-3:], '0\n\n')
+
+        self._box.add('From: foo\n\n0\n')  # Newline at the end
+        with open(self._path) as f:
+            data = f.read()
+            self.assertEqual(data[-3:], '0\n\n')
+
+
 class TestMMDF(_TestMboxMMDF, unittest.TestCase):
 
     _factory = lambda self, path, factory=None: mailbox.MMDF(path, factory)