Anonymous remailers are services that forward emails to an intended recipient 
while stripping the email of any identifying information. 
Typical emails contain headers in addition to the body of the message. These 
headers are used to pass the email from its originator to its destination, and
each server it passes through puts it's own stamp on it as it passes it along. 
In addition, it also contains the senders name and email address so that the
recipient knows who sent it. For example, take a look at the following headers:
X-F:  Fri Jun 02 01:34:25 2000
Received: from isp.com [199.29.xx.xxx] by 208.24.118.202 with ESMTP
This line tells me where the email came from - isp.com... if the email passes 
through more than one server, each one will stamp their own information on a new
line as they receive it. If there is more than one server, they will stack these 
line on top of each other, with the originating server at the bottom, and your email 
providers server at the top. This is where you would look to find out where to file
a complaint should you feel it is warrented. In this case, you would go to 
www.isp.com and look for an email address for their abuse department. Alternately 
you can just send your complaint to abuse@isp.com (most, but not all, ISP's use this 
email address for their abuse department).
(SMTPD32-4.06) id A57010F200EA; Fri, 02 Jun 2000 01:34:24 CDT
X-WM-Posted-At: isp.com; Fri, 2 Jun 00 02:24:22 -0400
X-WebMail-UserID: jonnydxxxxxx
This just shows that the user was using a webbased email service provided by isp.com
Date: Fri, 2 Jun 2000 02:24:22 -0400
This is the date and time the message was sent
Sender: Jonny Dxxxx 
From: Jonny Dxxxxxx 
Both of these lines (you will not usually see both lines in a typical header - it all
depends on the email server being used) tell the recipient who sent the message
To: "ThePsyko" 
Obviously, this line indicates to whom the email is intended
X-EXP32-SerialNo: 00000000
Subject: Re: [no subject]
Message-ID: <3941CBFC@isp.com>
Each email is stamped with a unique identifying ID code for easier tracking by the 
service providers (ISP's). This number can be handy if you have to file any complaints
to the ISP. There are other headers but you don't need to worry about them for now.
As you can see, the typical email header contains enough information to be able to not 
only file a complaint with the proper service provider, but also assists the provider 
in tracking which one of their hundreds of users sent the email. It is this information 
that is stripped from the email when an anonymous remailer is used. Recipients of 
anonymous emails can't tell WHO sent them or WHERE they came from. This is what makes
the remailers so popular, and also what seems to frighten those who are working towards
tightening up the internet. Below are some links to check out...