[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
> - 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 4.5.4/18.104.22.168 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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