Sat, 04 Oct 2008
/Admin/email/postfix:
Relay Your E-mail Through Multiple Servers with Postfix
Why would you want to do this? As mentioned in Simple Relay, if you
have a lot of e-mail going out and you are trying to relay all of it
through one poor free relay server, that relay server is probably going
to reach a threshold at which it labels you a "spammer" and starts
deferring or even bouncing your e-mails. So what I do is use my own
server to send e-mails directly to their destination as much as
possible. And for those domains that reject my direct connections
(theoretically because my server is running on a dynamic ip) I relay
through another server. Preferably I have several such "other" relay
servers so as to spread the load around and not have to deal with
bounced e-mail.
Postfix is not really designed to do this, so what I will present
here is a limited solution that will work under specific circumstances.
Basically the problem is that any free SMTP server I have been able
to find insists on authentication (logging in with a valid userid and
password) and further insists that the envelope address of the e-mail
you are sending must agree with the userid you are logging in with. For
instance, if you have a sina.com account with userid=mysinaname and
password=mysinapassword, if the envelope address is anything but
mysinaname@sina.com, the sina SMTP server will reject the e-mail. Note
that the "envelope address" is not necessarily the same as the "From:"
address in the e-mail's header, although they are usually the same.
If you are sending e-mail with an e-mail client, the client will of
course take care of all of this. But if you are sending e-mail through
your own local e-mail server, and then onwards to its destination
(optionally through another relay server) things get a little more
complicated as there may be e-mails being queued to the server from
multiple sources, and Postfix appears to have no way of associating a
given envelope address with a given destination address/domain.
Postfix is capable of routing e-mails to different relay servers
based upon their destination address. But it is left up to any software
queuing e-mail to Postfix for external delivery to take care of making
sure that the envelope address is correct for the relay server that
Postfix will later relay to. That is what makes this a limited solution.
Set Up Postfix
The solution lies in /etc/postfix/transport. Here is mine:
langex.net local
gmail.com smtp:smtp.sohu.com
hotmail.com smtp:smtp.sohu.com
sina.com smtp:smtp.sina.com
boltblue.com smtp:smtp.sina.com
msn.com smtp:smtp.sina.com
onlineworkshop.net smtp:smtp.sina.com
* smtp
Mail destined for langex.net never leaves my server and is delivered
locally. gmail.com and hotmail.com are both relayed via SMTP through
smtp.sohu.com. sina.com, msn.com, etc. are relayed via SMTP through
smtp.sina.com. Everything else ("*") goes out via SMTP directly to the
destination domain (no relay).
Ensure that /etc/postfix/sasl_passwd contains userid:password for
both smtp.sohu.com and smtp.sina.com, then run the following commands:
postmap /etc/postfix/sasl_passwd
postmap /etc/postfix/transport
/etc/init.d/postfix restart
Set Up PHP
My main source of e-mail on this server is a PHP-driven website. In
this website I have the following function:
function emailWrap($thisEmailAddress, $thisSubject, $thisMessage, $thisReplyTo)
{
$toDomain = stristr( $thisEmailAddress, "@" ); // extract domain from e-mail address
$toDomain = strtolower ( $toDomain ); // make all alphabetic characters lower case
switch($toDomain)
{
default:
$headMe = 'webmaster@langex.net';
break;
case "@gmail.com":
$headMe = 'bjlangex@sohu.com';
break;
case "@hotmail.com":
$headMe = 'bjlangex@sohu.com';
break;
case "@sina.com":
$headMe = 'langexnet@sina.com';
break;
case "@boltblue.com":
$headMe = 'langexnet@sina.com';
break;
case "@msn.com":
$headMe = 'langexnet@sina.com';
break;
case "@onlineworkshop.net":
$headMe = 'langexnet@sina.com';
break;
}
$headFrom = "From: Language Exchange Webmaster <" . $headMe . ">";
$headReplyTo = "Reply-To: " . $thisReplyTo;
$headContent = "Content-Type: text/plain; charset=GB2312";
$addheader = $headFrom . "\n" . $headReplyTo . "\n" . $headContent;
$addParam = "-f " . $headMe; // the PHP magic for setting the envelope address
mail($thisEmailAddress, $thisSubject, $thisMessage, $addheader, $addParam);
}
There are two main things to take note of in this function:
- The contents of the switch must be kept in alignment with
/etc/postfix/transport
- $addParam = "-f " . $headMe; is the statement that specifies/sets
the envelope address within PHP.
Re. item 2, I chased my tail for a long time before figuring out that
the "additional parameter" to the PHP mail()[1] function was the Linux
server way to do this. There is one particularly misleading PHP function
called ini_set()[2] which DOES NOT WORK on Linux, and is apparently
meant for Windows servers. In Summary
For those e-mail destination domains (hopefully the vast majority)
that do not give your server any grief, everything works just like a
default Postfix server, ie. Postfix connects directly to the destination
SMTP server and delivers the e-mail.
For those domains that regularly give you delivery problems for
whatever reason, if you can find a relay server that it will accept mail
from, then use that relay server.
But remember: any application on your server that queues an e-mail to
Postfix for one of these problem destination domains, where you have not
correctly configured the application to set the envelope address to
correspond to the relay server, will result in bounced e-mail. This is a
seriously non-trivial problem if you have other people using the server
and/or e-mail being sent from multiple different sources. If you can
limit the number of applications sending external e-mail to one, this
method works really quite well.
[1] http://www.php.net/mail
[2] http://www.php.net/ini_set
posted at: 03:38 | path: /Admin/email/postfix | permanent link to this entry
