Out-of-order mailing lists

James Wilkinson ubuntu at westexe.demon.co.uk
Thu May 5 12:39:33 UTC 2005


Tony Arnold wrote:
> Can e-mail messages on the same mailing list arrive out of order like
> they can on news? I'm not sure. It probably doesn't make much difference
> to the advice above anyway.

Yes, but it's unlikely.

E-mail is a best-effort medium anyway: there's no guarantee that any
message gets through.

In normal, light load, a user will send an e-mail. That will get nearly
instantly flipped from their PC to server to server to mailing list
server, and copies sent via other servers to everyone on the list.

Under heavier load, you'll start to see e-mails being queued at servers.
Most MTAs have limitations on the number of connections they'll have
open at any one time. Opening 1000 connections across a slow link won't
get the mail delivered much faster, but it will seriously slow down
interactive sessions (like SSH and the web) using the same link. And it
slows down the server (cache and disk issues).

These queues aren't necessarily first in, first out: in particular, if a
server has an open SMTP connection to another server, it may well send
all the e-mails it has for that server down the one connection.

But if someone sends large attachments to a number of recipients, then
later messages will be queued at that server for a while.

The important point here is that there isn't always just one way for
e-mails to go. Manchester University, for example, has four "MX hosts"
called mailrouter1 to mailrouter4. The DNS is set up so e-mail can go to
any of them. It's quite possible that a message going via mailrouter1
will overtake an earlier message stuck in a queue at mailrouter2.

Under an even heavier load, or with a down server, one server may not be
able to make a connection at all to the next server. At that point, it
will either keep the message itself and try again later, or forward it
to a backup server which will keep trying the e-mail at intervals.
So if a retry takes place just before the receiving server comes back to
life, it may take an hour or two before another send attempt is made,
during which time other e-mails will get through.

Hope this helps,

James.
-- 
E-mail address: james |  ... more holes in Internet Explorer than
@westexe.demon.co.uk  | Blackburn, Lancashire...
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