Re: [taugh.com-standards] Re: Gen-ART and OPS-Dir review of draft-ietf-appsawg-nullmx-06

[Viktor Dukhovni <[e-mail]>]

On Thu, Sep 04, 2014 at 12:37:50PM -0400, John R Levine wrote:

> If you get 521 as a server greeting it means "I'm not a mail server."  If
> you get 521 as a response to RCPT TO it means "That's not a mail domain."

This overloading is unfortunate.  It creates an implementation
challenge on the server side, because at least with Postfix, 421/521
responses can originate in milters, policy services, and access
tables.  In such cases the server interprets this as a "please drop
this client now" signal.

Since nullmx recipient policy might well be implemented in miters
and the like, Postfix has no way to distinguish between this new
proposed code (which seems to not be a "drop" signal) and all
previous uses which are.

Postfix also supports "soft_bounce", which downgrades all 5XX
replies to the corresponding 4XX replies.  However 421 after RCPT
TO does not carry a "That's not a mail domain, but try again later"
meaning.

The choice of 521 here seems rather unfortunate, and based on an
error the experimental RFC 1846.  Please consider 550 or similar.

-- 
	Viktor.