The mailbox classes based on _singlefileMailbox (mbox, MMDF, Babyl) implement the flush() method by writing the new mailbox contents into a temporary file which is then renamed over the original. Unfortunately, if another program tries to deliver messages while mailbox.py is working, and uses only fcntl() locking, it will have the old file open and be blocked waiting for the lock to become available. Once mailbox.py has replaced the old file and closed it, making the lock available, the other program will write its messages into the now-deleted "old" file, consigning them to oblivion.

I've caused Postfix on Linux to lose mail this way (although I did have to turn off its use of dot-locking to do so).

A possible fix is attached.  Instead of new_file being renamed, its contents are copied back to the original file.  If file.truncate() is available, the mailbox is then truncated to size.  Otherwise, if truncation is required, it's truncated to zero length beforehand by reopening self._path with mode wb+.  In the latter case, there's a check to see if the mailbox was replaced while we weren't looking, but there's still a race condition.  Any alternative ideas?

Incidentally, this fixes a problem whereby Postfix wouldn't deliver to the replacement file as it had the execute bit set.