Proxy Chaining
- Introduction
- Instructions

Proxy Chaining

Introduction

POPFile is an email proxy. In the simpliest possible installation, POPFile is an intermediary that passes along communications between your email client (e.g. Outlook) and your email server (e.g. pop.your_isp.com).

Simple POPFile installation
	Email client	→	POPFile	→	Email server
IP address	talks to	127.0.0.1	talks to	mail.isp.net
POP port	over port	110	over port	110

A mail client sends a request through POPFile to the mail server for email. The mail server responds by sending the email to POPFile which tags the email with categories or buckets and delivers them immediately to the mail client.

If you are using any additional email proxies with POPFile such as certain antivirus software, YahooPOPs, YPOPs! or Hotpopper, you need to configure them to work together in a daisy chained fashion like so:

Configuring POPFile to work with another proxy
	Email client	→	POPFile	→	(antivirus, etc.)	→	Email server
IP address	talks to	127.0.0.1	talks to	127.0.0.1	talks to	mail.isp.net
POP port	over port	123	over port	124	over port	110

This allows a virus checker to check email for viruses before they reach POPFile or your mail client or for YahooPOPs or YPOPs! to fetch mail from a Yahoo account. Note that you want to avoid conflicts with other proxies by either a) using different ports or b) a different IP address. However, a different IP address is often not practical for this particular application because your proxies (e.g. virus checker and spam checker) are running together on your local machine. If none of that makes sense, that's okay it isn't essential to using POPFile. Keep reading on.

The port you set in POPFile, the POP3 listen port setting on http://127.0.0.1:8080/configuration and in your email client (the pop or incoming server) are different than the setting your mail server uses (110 by convention) and the other proxy.

Instructions

The instructions below assume you have a mail proxy installed that inserts itself as a proxy like POPFile does. If you plan to install both an antivirus program and POPFile but have installed neither yet, install the antivirus software first according to the instructions provided by the author(s) of the software and then verify that you can a) get email and b) the antivirus software is working correctly. Then install POPFile following the directions below.

The most common example is probably an antivirus program. Some proxies act transparently, so the instructions will vary. See Norton Antivirus.

Note that before installing POPFile, the mail client contains the following entries made by the virus scanner:
```
pop3 server: 127.0.0.1
username: pop.isp.net:user
```
- where pop.isp.net is your mail server, user is your login name, and 127.0.0.1 indicates the “localhost” or computer that you are running your email client and virus software on.
- Although not stated explicitly, the above implies that the virus scanner listens on port 110. Generally the port for the mail server is specified elsewhere in your mail client, but 110 is the most common setting.
Install POPFile ( Windows installations or Cross platform installations).
Using http://127.0.0.1:8080/configuration, set the POPFile listen port to 123 and the POP3 host:port:user separator character: to a percent sign %.

Change the mail client's setup as follows:

pop3 server: 127.0.0.1
pop3 port: 123
username: 127.0.0.1%124%pop.isp.net:110:user

Note: If you are using POPFile 0.21.2 or before, use 127.0.0.1:124%pop.isp.net:110:user as username.

The above setup tells the mail client to contact POPFile on IP address 127.0.0.1 and port 123 (instead of the antivirus software as the email client did originally).
POPFile will examine the username and strip off the left side of the username (where the % is the division point), and use that to contact the virus scanner on 127.0.0.1 port 124, and pass the right side to the virus scanner as the username.
The virus scanner, listening on port 124, gets the username and strips it's left side off (remember it uses : as its separator), getting the address pop.isp.net to contact the real mail server on port 110, using the right side username of user.

So the general format for using two chained proxies, POPFile, and a mail client is:

proxy[%port]%pop.mailserver.net[:port]:username

where:

The text inside ”[ ]” is optional depending on the situation.
The % as a center line; everything to the left of the percent sign is the left side; everything to the right is the right side.
The left-side entry should be the first proxy in the chain relative to the mail client.
The chain should end at the real mail server.
The left side needs to be formatted for POPFile while the right side needs to conform to whatever that next proxy wants to see.