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.
Philip Guenther