Backport 58618:

Issue 1307 by Derek Shockey, fix the same bug for RCPT.

Lib/smtpd.py

@@ -237,7 +237,7 @@ class SMTPChannel(asynchat.async_chat):
         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