+The text is not included in the response to an EXPN command. In non-SMTP cases
+the text is included in the error message that Exim generates.
+
+.cindex "SMTP" "error codes"
+By default, Exim sends a 451 SMTP code for a &':defer:'&, and 550 for
+&':fail:'&. However, if the message starts with three digits followed by a
+space, optionally followed by an extended code of the form &'n.n.n'&, also
+followed by a space, and the very first digit is the same as the default error
+code, the code from the message is used instead. If the very first digit is
+incorrect, a panic error is logged, and the default code is used. You can
+suppress the use of the supplied code in a redirect router by setting the
+&%forbid_smtp_code%& option true. In this case, any SMTP code is quietly
+ignored.