Index: Lib/smtpd.py
===================================================================
--- Lib/smtpd.py	(revision 58606)
+++ Lib/smtpd.py	(working copy)
@@ -237,7 +237,7 @@
         if not self.__mailfrom:
             self.push('503 Error: need MAIL command')
             return
-        address = self.__getaddr('TO:', arg)
+        address = self.__getaddr('TO:', arg) if arg else None
         if not address:
             self.push('501 Syntax: RCPT TO: <address>')
             return