Re: your mail

[Brian Candler <B.Candler at pobox.com>]

On Tue, May 29, 2001 at 11:14:31AM +0200, Antonio Godinho wrote:
> You can also run the ETRN implementation which is found on UNIX 
> or even other windows mailers nowadays including Exchange. It 
> just uses a regular PPP connection to poll the mail directly from 
> the mail queue of the server you are connecting to.

Most ETRN implementations at the server side are hugely inefficient. The
ones I have seen involve setting sendmail to 'queue only', then when user
example.com dials in, running
    sendmail -qR  at example.com

This is horrible because:
(1) sendmail has to scan through the mails for _all_ customers to extract the
ones for the customer in question; this scales very badly.
(2) the customer needs to have a static IP address dialup, which also scales
extremely badly. This is for two reasons - so that sendmail can follow MX
records (or mailertable entries), and because ETRN doesn't do any
authentication.

A better solution to (1) would be to deliver the mail to a mailbox as usual,
and the run a second program which takes the mailbox and resends it using
SMTP. I have managed to hack together qmail's serialmail to do this; I will
document this if there is any interest on this list. Maybe a simpler method
is to use 'fetchmail' which opens a POP3 connection, downloads the mail and
converts it to SMTP. The problem with both these approaches is that there is
no standard way of keeping the envelope recipient in a POP3 mailbox. You can
configure Exim to add a 'Delivered-to:' header, but then you need to
configure the other bits and pieces to use it.

As for (2), the problem with authentication can be solved by using a strange
looking ETRN string (for example, 'ETRN #example.com#password') and
validating it at the server, or by requiring the client to authenticate
using SMTP AUTH before issuing ETRN. But even then, it will cause a second
TCP connection to be opened to the recipient, and there is a danger that the
client will hang up, a second person will dial in and get the same IP
address, and the mail will be delivered to the wrong customer.

This problem in turn should be solved by the ATRN extension to SMTP
(RFC2645), where a TCP connection to port 366 is turned around to return
mail, together with SMTP AUTH (RFC 2554), but these extensions are not yet
widely implemented in clients, and I have not yet worked out a good way of
implementing this at the server side.

Regards,

Brian.

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