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List:       exim-users
Subject:    Re: [exim] cutthrough deliver/mode
From:       Jeremy Harris via Exim-users <exim-users () exim ! org>
Date:       2020-05-14 22:20:03
Message-ID: 2eecd64c-01fd-a640-44e6-7ebf4d7e7eb3 () wizmail ! org
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On 14/05/2020 10:08, kevin shell via Exim-users wrote:
> I read through the exim spec and came cross cutthrough deliver/mode,
> Can someone explains what is cutthrough deliver mode?
> I can't find a definition in the exim spec doc.

Cutthrough delivery sets up and uses an SMTP connection
for onward delivery while the original connection from
the sender is still up.  The overlap means that a refusal
to accept the message can be communicated directly back
to the source.  In turn that means that data-time scanning
(eg. for malware) by the destination system does not
result in a need to send a DSN to the original sender.
So backscatter to innocents forged by spammers does not
occur.

Essentially it's an extension of recipient verify callouts.

Contrast cutthrough delivery with store-and-forward delivery,
which is the traditional mode.


There's a short description in the ACL chapter of the spec.



Ethernet switches are often designed to do cutthrough at
the ethernet frame level.
-- 
Cheers,
  Jeremy

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