When opening mailboxes the module checks for errno.EACCES. This doesn't help if the location is mounted read-only. Something like the following (against Python 2.6) would fix it, there are a few other checks in mailbox.py for EACCES too.

--- mailbox.py.orig	2011-02-24 15:02:22.000000000 +0800
+++ mailbox.py	2011-02-24 15:02:05.000000000 +0800
@@ -512,7 +512,7 @@ class _singlefileMailbox(Mailbox):
                     f = open(self._path, 'wb+')
                 else:
                     raise NoSuchMailboxError(self._path)
-            elif e.errno == errno.EACCES:
+            elif e.errno == errno.EACCES or e.errno == errno.EROFS:
                 f = open(self._path, 'rb')
             else:
                 raise

Creating a test for this may not be practical :(

On Thu, Feb 24, 2011 at 03:07:10PM +0000, R. David Murray wrote:
> Creating a test for this may not be practical :(

Then should we just go ahead with this change as it seems reasonable
one.

-            elif e.errno == errno.EACCES:                                                                                                                             
+            elif e.errno == errno.EACCES or e.errno == errno.EROFS:

Committed to py3k in r88730, 3.2 in r88731, and 2.7 in r88732.

I tested it by hand; mbox open failed before patch, succeeded (read-only) after patch.  I also changed two other cases of EACCES tests without testing them; I don't see how it could hurt to add EROFS.  I did not touch the os2-specific cases as there it looks like EROFS might require a different semantic from EACCES.