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From Martin Brown <martin.br...@3bview.com>
Subject Re: James and Postini
Date Wed, 02 Sep 2009 10:35:31 GMT
Thanks for your response.

> > We've hit an issue when sending via Postini from James.
> >
> > The destination email system has two servers listed in DNS, one with
> > cost 10 and the other with cost 20. The server with cost 10 does not
> > accept connections. The other appears ok.
> >
> > Postini don't spool email - they operate as a thin proxy. Therefore when
> > the connection fails Postini send back a 400 code. The intention is that
> > the originating email server retries the send immediately on the same
> > connection. Postini will then try to deliver the email to the higher
> > cost MX server.
> 
> a 400 code can't be used when the "intention is that the originating
> email server retries the send IMMEDIATELY", instead a 400 means that
> temporarily this cannot be delivered, retry later (I don't know any
> MTA that retries immediately after a 400).

Are there any codes that would indicate an immediate retry? 

Our customer reports that if James is removed from the email flow, so
that email goes directly from Exchange to Postini (see diagram below),
then email is delivered correctly.

According to Postini:
The service supports all common mail servers, including Microsoft
Exchange Server, Lotus Domino, Postfix, Sendmail, or Novell Groupwise.

I can't see anything in the RFCs that allows Postini to work in this
way, but nor can I see them changing.

> > James operates according to the RFCs and queues the message for a later
> > delivery attempt. However, since the later delivery is on a new TCP/IP
> > connection, Postini has lost the state and retries delivery to the lower
> > cost MX server. Thus delivery can never succeed.
> 
> AFAIK James, when receiving a 400 from the MX 10 will try to connect
> to the MX 20 and only after both fail temporarily it will put the mail
> in spool for a later retry. When a retry is needed it will start again
> with an attempt to MX 10 followed by an attempt to MX 20. So both
> servers should be tried multiple times, until one either send a
> permanent failure or enough attempts have been made.

Sorry - I probably didn't make this clear. The problem exists when
Postini is filtering outgoing email rather than the incoming side. Thus
James is configured to use Postini as a Smarthost/Gateway, so no MX
lookup happens within James. The MX lookup happens within Postini, but
Postini relies on the sending MTA retrying the send to make this work.


The flow is like this:
                             DNS MX Lookup
                             |
                             |
Exchange ---> James ---> Postini ---> Destination-10
                                  \
                                   -> Destination-20

James is processing the outgoing email - it does not run any IMAP/POP
services - email is SMTP in and SMTP out.

The installation is at a customer so we cannot simply turn off Postini.


Thanks for your help,

Martin

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