POP3, IMAP, and downloading subject lines.

James Wilkinson james at westexe.demon.co.uk
Thu Feb 3 13:31:23 UTC 2005


Mike Hoy wrote:
> why i think thunderbird sucks..
> 
> i have  filter set up to kill this "why i think FC3 sucks!" thread and a 
> few others. but thunderbird only applies the filters once the emails are 
> downloaded. So it's a nice way to put all of your emails away when 
> you're done reading them but not for killing threads like this one.

Unfortunately, it's logicaly impossible to kill e-mail threads by
subject until the mail client has seen the subject!

In standard Internet e-mail, the subject line is part of the e-mail
header.

Most ISPs use POP3 for client-bound e-mail. This is a very simple
Internet protocol. Normally a client simply downloads an entire e-mail
(including headers), and by the time the client has worked out that the
subject isn't wanted, then it's too late: the rest of the e-mail is
already coming!

It is possible to use the TOP command to merely get the message headers,
but this is listed as "optional": not all servers support it. More
importantly, I believe that there is no standard way to get the rest of
the message *without* getting the headers as well. So you'd have to
download the headers, check that you want the e-mail, then download the
headers and the rest of the e-mail. On a mailing list like this, for
example, unless you kill lots of threads, then you're going to be
downloading the headers twice for nearly every e-mail, and you actually
download more.

With SMTP, the oldest standard protocol, there is no way to interrupt an
inbound message once it's started coming while still telling the sending
mail server not to try again.

IMAP is a much newer and much richer protocol. It's also not much used
by ISPs because it supports keeping messages on the server, and the ISPs
want the e-mail off their servers as quickly as possible!

It does have support for retrieving subjects, and even for getting the
server to search by subject and delete the results. I can't see any
indication (quickly checking the RFC) that this is optional, but I don't
know how much client software supports it: anyone?

That's rather longer than I expected. Sorry about that.

James.
-- 
James Wilkinson       | Telsa ruthlessly depopulates the cuddly toy pile.
Exeter    Devon    UK | There is only room for so many penguins in any one
E-mail address: james | house. Alan wakes. Telsa throws cuddly toys down the
@westexe.demon.co.uk  | stairwell. Alan is struck by a blinding flash of
                      | penguin.  -- Telsa Gwynne's Diary.




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