Index: mailer.py
===================================================================
--- mailer.py	(revision 28773)
+++ mailer.py	(working copy)
@@ -1219,12 +1230,12 @@
           raise UnknownMappingSection(sectname)
         # construct a lambda to look up the given value as an option name,
         # and return the option's value. if the option is not present,
-        # then just return the value unchanged.
+        # then either return the _default_ option and if that fails, return the
+        # value unchanged.
         setattr(self.maps, optname,
                 lambda value,
-                       sect=getattr(self, sectname): getattr(sect,
-                                                             value.lower(),
-                                                             value))
+                       sect=getattr(self, sectname):
+                            getattr(sect, value.lower(), getattr(sect, '_default_', value)))
         # mark for removal when all optnames are done
         if sectname not in mapsections:
           mapsections.append(sectname)