Custom mail delivery solutions

Contents:

First, a recap of the basics. How does mail from far away get to your account - say - somebody@my-domain.com ?

The remote mail client probably punts the mail to a smarter host - the SMTP server of their ISP ( Internet Service Provider ). this will have been configured when first setting up their mail client.
Their ISP mail server will ask a local nameserver for the Mail Exchanger records of my-domain.com. These are machines listed as handling mail for this domain. The ISP will try to deliver directly to the lowest-numbered MX record in the list, via SMTP. If it cannot connect, it will try other MX hosts, usually in ascending order.
In this last step, or via secondary MX hosts, the mail will eventually land on the target host - the lowest-numbered MX, often your local ISP you have contracted for this service.
Most of the time, your ISP will dump all the mail for that domain into a single mailbox, and leave you to figure the rest. If this is the case, your best, and only, solution below is fetchmail.
Fetchmail uses POP3 or IMAP to get mail from your server. Fetchmail's limitations are mostly to do with the fact that the SMTP envelope information has been discarded on delivery to your mailbox, though it has some creative (but not foolproof) ways to try to get around the problem. See the section on THE USE AND ABUSE OF MULTIDROP MAILBOXES in the fetchmail documentation.
Maybe your ISP is a little more cooperative. If you can get a fixed IP address on dialup, you could list them as a Mail Exchanger, but not the lowest, and yourself as the lowest. This will only work if you have a fixed IP. See Fixed IP solutions below.
Much of the messiness of this whole problem is that SMTP is not authenticated, and therefore relies on name services - fixed IP addresses, to solve this. One way to preserve the SMTP protocol across dynamic IP addresses is to batch SMTP commands in a file, and pass the file over a separate transport. Batched SMTP is similar to normal SMTP, but, because it is being written to a file, means that all responses are ignored. Fortunately this is not traumatic - if the protocol is written to a file assuming that all responses will be OK it all works. See Batch SMTP below.

Fetchmail configuration

Fetchmail by Eric S. Raymond esr@snark.thyrsus.com

Best to read the fetchmail docs. Fetchmail by default delivers the mail to a local SMTP server. Be careful to tell that local SMTP server to treat your domain as local, otherwise it may punt the mail to your local ISP as a smarthost (as you should be doing with all your other mail) and you will have a mail loop.

For a single user Linux box, I prefer to configure fetchmail with the '--mda' (mail delivery agent) option, pointed to procmail, so I can leave my SMTP server (if I run one - why bother ?) in a basic smarthost configuration.

Administrative access to SMTP server

If You control the machine to which the mail is delivered, in particular if you control the SMTP server configuration, you have other options. One of the easiest options is to add a custom header for the envelope recipient so that fetchmail can reliably do its thing (adding X-Envelope-To: header, for instance) and continue with fetchmail.
Still with us, eh ? To look past here, you must be able to edit your SMTP server configuration file. If you are the administrator of the machine, and the machine only handles your mail, I recommend you install either exim or qmail as your SMTP server. qmail is very capable, but I do not cover it here. There are better help texts available on the qmail site. For exim see below.
Maybe you have an account on a 'virtual server' that runs daemon services chrooted in your home directory on your behalf. I have used these, and they have always run sendmail. Use a custom X-Envelope-To: header and use fetchmail .. really.

Fixed IP solutions

Do you have a fixed IP address ? If you do not, it is worth a little time to see if you can get one. My ISP gave me a fixed IP address even though I dial up to their general modem pool - Great !! I think that is a feature of their Portmasters.

A private line will usually get you a fixed IP address.

If you can get a fixed IP address on dialup, you could list them as a Mail Exchanger, but not the lowest, and yourself as the lowest. The Sendmail ETRN command will tickle the remote SMTP server to deliver your mail after dialing up.

Sendmail ETRN command

You need a recent version of sendmail at your server (ISP). Less than 8.8.x will not work. A simple way to find out if your server has the requisite software is to :-

telnet mail.my-isp.net 25
HELP
QUIT

You are looking for ETRN as a valid command. Sendmail can be found at www.sendmail.org along with all sorts of other useful sendmail stuff.

You now need to set up your MX records for proper delivery of mail. It will eventually look something like this. For your domain "my-domain.com", and your ISP "my-isp.net" :-

$ nslookup -type=mx my-domain.com
Server: ns.my-isp.net

Non-authoritative answer:
my-domain.com preference = 10, mail exchanger = my-domain.com
my-domain.com preference = 20, mail exchanger = mail.my-isp.net

This means that if you are online, mail for your domain will be delivered directly to your machine. If you are offline, it will be spooled at mail.my-isp.net. When your connection comes up, execute the following simple script :-

#! /bin/sh
#
# Send ETRN command to sendmail 8.8.x - Andy Rabagliati <andyr@wizzy.com>
#
telnet mail.my-isp.net smtp <<SMTP_EOF
ETRN my-domain.com
QUIT
SMTP_EOF
#
# End of Shell script
#

And Bob's your uncle, as we say. I used to have a perl / finger script that would do this, but it was perl4, and past its time ..

There is a good description of other alternatives at http://www.swinc.com/resource/exch_dq.htm though those intructions are aimed at MS Exchange users.

Next Batched mail solutions