[sclug] duplicate mail

[Alex Butcher]

On Wed, 27 Jun 2007, Graham wrote:

> Heres my timeout theory:
>

[...]

> + .
>
> <-- here the sender is waiting
> <-- for the sever to say "250 ok: Message 1234 accepted"
> <-- here the server is overloaded by SPAM from absent taiwanese friends
> <-- so it takes more than 2 minutes to respond to the DOT
> <-- That client was tested without SO_KEEPALIVE ((guess))
> <-- Or some other network drop (such as NAT firewall)
> <-- So when the socket is dropped the client didnt get the 250 OK
> <-- and it then tries MX-TWO, immediately or later
> <-- MX-ONE has succeeded, and sends 250 OK but too late
> <-- MX-ONE cannot then undo its own send, or queue-action

This is indeed one mechanism by which SMTP messages can end up duplicated
these days; my employer's mail wrangler warned this might happen when I was
suggesting ways of solving a related problem.

Best Regards,
Alex.
-- 
Alex Butcher, Bristol UK.                           PGP/GnuPG ID:0x5010dbff

"[T]he whole point about the reason why I think it is important we go for
identity cards and an identity database today is that identity fraud and
abuse is a major, major problem. Now the civil liberties aspect of it, look
it is a view, I don't personally think it matters very much."
  - Tony Blair, 6 June 2006 <http://www.number-10.gov.uk/output/Page9566.asp>