--On Monday, 23 April, 2007 14:12 -0600 Philip Guenther
<guenther+smtp(_at_)sendmail(_dot_)com> wrote:
On Mon, 23 Apr 2007, Hector Santos wrote:
...
This 4.3.2 sentence makes it clear that "1yz " is not valid,
Unless extended using the mechanisms described in Section
2.2, SMTP servers MUST NOT transmit reply codes to an SMTP
client that are other than three digits or that do not
start in a digit between 2 and 5 inclusive.
But it is also inconsistent with the 4.2 sentence:
An SMTP server SHOULD send only the reply codes listed in
this document.
Those two statements are not actually inconsistent. The
first, from 4.3.2, only restricts what the server can use for
the first digit of a reply code. The second is a (looser)
restriction on the entire reply code. For example, a server
that used the reply code "472" would comply with the 4.3.2
MUST, but since "472" is not listed in RFC 2821, the server
would not be following the recommendation of the SHOULD from
4.2.
FWIW, that has been my understanding since long before 2821.
john