Index: Lib/mailbox.py
===================================================================
--- Lib/mailbox.py	(revision 52779)
+++ Lib/mailbox.py	(working copy)
@@ -1887,7 +1887,8 @@
 def _sync_flush(f):
     """Ensure changes to file f are physically on disk."""
     f.flush()
-    os.fsync(f.fileno())
+    if hasattr(os, 'fsync'):
+        os.fsync(f.fileno())
 
 def _sync_close(f):
     """Close file f, ensuring all changes are physically on disk."""