Application Note: How to route your outgoing mail through your Internet Service Provider's mail servers

Overview

This document contains setup notes for various mail servers and gateways commonly found running on dial-up networks. One thing to remember that applies to each of these: The server settings you specify in your mail relay do not affect the From: or Reply-To: addresses you specify in your mail client software.

Some examples feature a Masquerade option that might be useful if your Internet provider refuses to deliver mail with an unknown domain name in the Reply address. This still does not change the reply address that your intended recipients will see. Mail servers not specified here should have an obvious "Gateway" or "Relay" or "Firewall" setting that lets you specify a server to forward outbound mail to.

Mail servers and gateways covered in this document:

• Sendmail (8.8.x and up)
• Whistle InterJet
• Deerfield's MDaemon
• Microsoft Exchange Server
• EMWAC IMS and Rockliffe MailSite
• Postfix
• Qmail
• Novell Groupwise
• ZMailer

Sendmail

Add these lines to your sendmail.mc file:

define(`SMART_HOST',`mail.yourisp.net')dnl <--- Your ISP's mail server name goes here
MASQUERADE_AS(yourisp.net)dnl  <--- Your ISP's domain name goes here
FEATURE(`allmasquerade')dnl
FEATURE(`masquerade_envelope')dnl

These instruct Sendmail to direct all outgoing mail through "mail.yourisp.net" or whatever you specify as your outgoing mail server.

The "masquerade_as" setting will change the sender envelope (MAIL FROM: contents, also known as the Envelope Sender) in case your ISP's outgoing server denies outbound mail with different domains in the MAIL FROM: or Envelope Sender line.

Suresh Ramasubramanian has more to say on Sendmail on dial-up lines:

There are a couple of other things if the server's on a dialup (like the ability to queue mail on the local box when offline). I've written a short howto at http://www.hserus.net/dlhowto.html which explains how to configure a linux box to be used on a dialup. The specific page dealing with sendmail config is at http://www.hserus.net/pop_smtp.html.

I'd be honored if this is included in your docs.

Glad to be of some use. Also, sorry for the delays and no, I'm not going to put your email address here just so it can be strip-mined by spammers. :-)

For specifics see Red Hat Software, http://www.redhat.com/support/resources/faqs/RH-sendmail-FAQ/c20.html and sendmail.org, http://www.sendmail.org/.

Whistle's InterJet

On the InterJet's Mail Agent / Options configuration pages, type your ISP's outgoing server name in the "forward all mail to:" field.

Also, upgrade your InterJet's firmware to 2.1p2 or later to prevent relay theft by spammers. See Whistle's tech noteon preventing spam relay.

Deerfield's MDaemon

We most commonly see MDaemon used in tandem with Wingate to provide email and general Internet connectivity. If this is your case, you should also make sure your Wingate proxy is not accessible from outside your LAN (such that spammers can't access your ISP's mail server via your proxy!) One way to accomplish this is programming Wingate not to proxy for connections on Port 25. You can use MDaemon to relay your mail instead.

Choose the Setup / Primary Domain menu in MDaemon's configuration program. Look for a Domain / Gateway dialogue that lets you specify wether MDaemon handles mail delivery itself or instead forwards the mail to another server. Specify your Internet provider's normal outgoing mail server as the gateway host.

For specifics see Deerfield's KB article 1396, http://www.deerfield.com/support/MDaemon/kb/index.htm?a=1396

Microsoft Exchange Server

Much of this applies to Exchange Server 5.5 or later. You may need to upgrade older versions, especially to prevent relay theft by spammers.

The Exchange Server Administrator program has a Connections tab that lets you specify how to route mail for specific domains (Message Delivery). It also has a default route. Select "Forward all messages to host:" and type in the name of your Internet provider's outgoing mail server. You can override this rule for specific domains (such as your own).

For specifics see this Exchange Server how-to:
http://support.microsoft.com/default.aspx?scid=/support/Exchange/Content/HowTos/dialupims.asp.

EMWAC IMS and Rockliffe MailSite

You should upgrade to IMS 0.83 or later if you're running the freeware IMS or one of its clones, and also install MFILTER and Antirelay to prevent relay theft. You will find these tools and dial-up scripts at http://www1.sica.com/ims/. MailSite is already up to date.

Look for the Domains tab in the IMS control panel. Type yout Internet provider's outgoing mail server name in the SMTP Gateway box.

Postfix

Add these lines to your mail.cf file:

mydomain = yourisp.net
myorigin = $mydomain
relayhost = mail.yourisp.net

The myorigin parameter specifies the domain that appears in mail that is posted on this machine. The above setting forces it to be the domain name of your ISP instead of the full name of your system, and works with mail servers that permit relay based on domain name (much like Sendmail's Masquerade feature). The relayhost parameter routes all outgoing mail through the mailserver of your ISP.

Postfix has a FAQ which includes how to use it on dial-up connections, http://www.postfix.org/faq.html.

Thanks to Furio from spin.it.

Qmail

Qmail takes explicit routes from the file /var/qmail/control/smtproutes. If your ISP's mail host is smtp.yourisp.net, put in it a line containing

:smtp.yourisp.net

(The part before the colon is the domain to route through that server, with nothing before the colon meaning to make it the default.)

You may also want to put your ISP's domain name into /var/qmail/control/defaulthost so that name appears as the domain part of From: and envelope "mail from" addresses. The defaulthost setting lets you work with mail servers that permit relay based on domain name. You can also control the outgoing domain with environment variables, see the man page for qmail-inject.

Courtesy of John Levine. See http://www.qmail.org/ for more resources.

Novell Groupwise 4.11 or later

Look for GWSMTP.CFG and add this line to it:

/mh smtp.yourisp.net

This instructs Groupwise to direct all outgoing mail through "smtp.yourisp.net". See Novell's article ID 2928239. http://support.novell.com/

ZMailer

Ambrose Li provides this config information for ZMailer, http://www.zmailer.org/ :

Routing in ZMailer is controlled by the MAILVAR/routes file. If you need to route all your mail to your ISP's server, you can put the following line into this file: (MAILVAR is defined in /etc/zmailer.conf, usually.)

.	smtp.yourisp.net

(Replace smtp.yourisp.net with your mail server's host name.)

For recent versions of ZMailer you will need to rebuid the routes map using the "zmailer new-routes" command. For very old versions of ZMailer you will have to make sure the routes file is sorted (if your routing table contains more than one line), but in this case you probably should consider upgrading.