From: Victor Duchovni (Victor.DuchovniMorganStanley.com)
Date: Fri Apr 06 2007 - 13:52:08 CDT

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On Fri, Apr 06, 2007 at 02:39:59PM -0400, Wietse Venema wrote:

> > This turned out to be incomplete information. What I finally concluded was
> > that email needed to be retrieved before email can be sent through the smtp
> > host. Sending email is disabled until the retrieval is done. There is also a
> > timeout, such that after a short length of time with no email activity,
> > retrieving email will be required again before email can be sent. The hosting
> > company said that the timeout was approximately 5 minutes, but the person I
> > was talking to really did not know the exact length of the timeout.
>
> In that case you need a mechanism that disables Postfix's email
> deliveries until something tells Postfix to deliver mail.
>
> Postfix has such a mechanism, but it does not distinguish between
> destinations. That is good enough for most people who have only
> one mail relay host.
>
> /etc/postfix/main.cf:
> defer_transports = smtp
>
> To deliver smtp mail, use the command:
>
> sendmail -q
>
> But this does not work because you use sender_dependent_relayhost_maps.
>
> The following would hack it, but does not yet exist:

One can use indirection and multiple instances to bootstrap nexthop
diversity into transport diversity.

        input instance ---> various output instances ---> destination

Use sender_dependent_relayhost_maps to inject to the right output instance
([127.0.0.1]:some_port). The various output instances can have appropriate
deferred transports.

Admittedly more complex, but doable today without new Postfix code. The
problem with the sender also choosing the transport, is that local
deliveries don't work right. This is why sender dependent routing was
abandoned in favour of sender dependent relayhost.

It looks to me that the real feature being sought is perhaps the ability to
defer all mail for a given transport:nexthop, rather than the entire
transport:

    defer_transports = smtp:[some.relay]

this would not be too hard to add, and works sensibly with both local
and remote recipients..

--
        Viktor.

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