diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 325b9c9..7a29555 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -1462,9 +1462,10 @@
 
     def _become_message(self, message):
         """Assume the non-format-specific state of message."""
-        for name in ('_headers', '_unixfrom', '_payload', '_charset',
-                     'preamble', 'epilogue', 'defects', '_default_type'):
-            self.__dict__[name] = message.__dict__[name]
+        type_specific = getattr(message, '_type_specific_attributes', [])
+        for name in message.__dict__:
+            if name not in type_specific:
+                self.__dict__[name] = message.__dict__[name]
 
     def _explain_to(self, message):
         """Copy format-specific state to message insofar as possible."""
@@ -1477,6 +1478,8 @@
 class MaildirMessage(Message):
     """Message with Maildir-specific properties."""
 
+    _type_specific_attributes = ['_subdir', '_info', '_date']
+
     def __init__(self, message=None):
         """Initialize a MaildirMessage instance."""
         self._subdir = 'new'
@@ -1584,6 +1587,8 @@
 class _mboxMMDFMessage(Message):
     """Message with mbox- or MMDF-specific properties."""
 
+    _type_specific_attributes = ['_from']
+
     def __init__(self, message=None):
         """Initialize an mboxMMDFMessage instance."""
         self.set_from('MAILER-DAEMON', True)
@@ -1699,6 +1704,8 @@
 class MHMessage(Message):
     """Message with MH-specific properties."""
 
+    _type_specific_attributes = ['_sequences']
+
     def __init__(self, message=None):
         """Initialize an MHMessage instance."""
         self._sequences = []
@@ -1769,6 +1776,8 @@
 class BabylMessage(Message):
     """Message with Babyl-specific properties."""
 
+    _type_specific_attributes = ['_labels', '_visible']
+
     def __init__(self, message=None):
         """Initialize an BabylMessage instance."""
         self._labels = []
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 95f8e0d..0fe4bd9 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -1330,6 +1330,14 @@
         # Initialize with invalid argument
         self.assertRaises(TypeError, lambda: self._factory(object()))
 
+    def test_all_eMM_attribues_exist(self):
+        # Issue 12537
+        eMM = email.message_from_string(_sample_message)
+        msg = self._factory(_sample_message)
+        for attr in eMM.__dict__:
+            self.assertTrue(attr in msg.__dict__,
+                '{} attribute does not exist'.format(attr))
+
     def test_become_message(self):
         # Take on the state of another message
         eMM = email.message_from_string(_sample_message)
@@ -1596,6 +1604,21 @@
         for class_ in self.all_mailbox_types:
             self.assertRaises(TypeError, lambda: class_(False))
 
+    def test_type_specific_attributes_removed_on_conversion(self):
+        reference = {class_: class_(_sample_message).__dict__
+                        for class_ in self.all_mailbox_types}
+        for class1 in self.all_mailbox_types:
+            for class2 in self.all_mailbox_types:
+                if class1 is class2:
+                    continue
+                source = class1(_sample_message)
+                target = class2(source)
+                type_specific = [a for a in reference[class1]
+                                   if a not in reference[class2]]
+                for attr in type_specific:
+                    self.assertNotIn(attr, target.__dict__,
+                        "while converting {} to {}".format(class1, class2))
+
     def test_maildir_to_maildir(self):
         # Convert MaildirMessage to MaildirMessage
         msg_maildir = mailbox.MaildirMessage(_sample_message)
diff --git a/Misc/NEWS b/Misc/NEWS
index 6256e02..4c3758d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -19,6 +19,9 @@
 Library
 -------
 
+- Issue #12537: The mailbox module no longer depends on knowledge of internal
+  implementation details of the email package Message object.
+
 - Issue #7978: socketserver now restarts the select() call when EINTR is
   returned.  This avoids crashing the server loop when a signal is received.
   Patch by Jerzy Kozera.