[Greylist-users] RE: Broken MTA? - Exchange 2003 SMTP

Robert L Mathews lists at tigertech.com
Wed Jan 19 14:56:44 PST 2005

Bob de Wildt <bob.dewildt at cysonet.com> wrote:

> We have experimented with the codes and we took the following RFC's to
> hand:
> - RFC 821 (aug 1982)
> - RFC 1891 (jan 1996)
> - RFC 1893 (jan 1996)
> - RFC 3463 (jan 2003).
> According to all of them the 451 status code leaves room for any MTA to
> break the connection and return the message immediatly.

Oops. You missed RFC 2821, which is the one that covers this. It 
explicitly says that any 4XX code MUST be retried.

It first defines any 4XX code as a "Transient Negative Completion 
reply", then goes on in section 4.2.5 to say that an SMTP server that 
accepts a message...

"accepts responsibility for:

-  delivering the message (if the recipient mailbox exists), or

-  if attempts to deliver the message fail due to transient
    conditions, retrying delivery some reasonable number of times at
    intervals as specified in section 4.5.4."

And then section 4.5.4/ says "mail that cannot be transmitted 
immediately MUST be queued and periodically retried by the sender." It 
goes on to recommend retrying for several days.

So there is no doubt that all mail servers receiving any 4XX reply must 
"periodically" retry it. Anything else is a violation of RFC 2821.

I'm fairly sure that many people using Exchange servers have been able 
to send us mail with no problem, despite greylisting. Are you sure that 
what you're seeing isn't just a single Exchange server with a sysadmin 
who set the retry time to zero, not understanding that this would cause 
all sorts of problems (in addition to greylisting)?

Robert L Mathews, Tiger Technologies    http://www.tigertech.net/

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