CleanMail - Transparent SMTP/POP3 Proxy
Another secret of CleanMail's flexibility is the highly versatile filter pipeline. It is designed to provide an open interface that allows integrating a wide range of third-party tools, e.g. for email classification, automated mail processing, server-based email encryption, or virus detection.
CleanMail submits incoming mails to a series of mail filters, the so-called filter pipeline. Examples of mail filters are the attachment filter, anti-virus blockers, or the SpamAssassin filter.
Each filter analyzes the message and returns a result deciding how a mail is further processed. Messages can be delivered, tagged as junk, deleted, or redirected to another account.
A sophisticated scheduler allows some filters to execute at the same time in parallel, while others are executed sequentially. The filter order matters: To help conserve resources and to increase throughput, filters lower down in the filter pipeline are not invoked if an earlier filter has already decided that a mail should be deleted.
To learn more about the filter pipeline, about filter types and their effectiveness, please see the CleanMail Filter Tuning Howto.
Transparent Proxy Technology
CleanMail can be configured as a proxy for SMTP (CleanMail Server version only) and POP3 servers. In this configuration, it accepts incoming SMTP or POP3 connections and forwards all requests to your existing mail software while scanning incoming mail for spam or viruses. This approach has several benefits:
- Most of your mail server's settings remain in effect. If you choose not to accept mails to certain recipient addresses, CleanMail won't accept it either. If you use SMTP AUTH, you can still use it after CleanMail has been installed.
- CleanMail automatically supports the same SMTP or POP3 extensions as your mail server.
- If your mail server is up and running, there is little change needed in its configuration to make it work with CleanMail.
- CleanMail does not implement the overhead of spooling mails until they can be delivered to your mail server. This eliminates potential problems before they occur, and makes CleanMail simple and straightforward to use.
CleanMail never accepts mails on its own volition, it always makes sure a mail has been accepted by your mail software before acknowledging a successful mail transmission.
Stop Spam & Viruses Before They Hit Your Mail Server
The best place to stop SPAM is before it hits your SMTP server. Always prefer filtering mail at the SMTP level, for two reasons:
- Spam mails can be deleted outright, before they enter your system. This saves your money, as you need less storage, bandwidth, and less of your users' time.
- If a legitimate email is identified as spam (false positive), the sender will be notified that his message might not be read by the recipient.
Mail Filtering Policies
CleanMail provides a lot of different Mail Filtering Policies, which enables each customer to configure it according to their respective corporate policies.
Mail Filtering Policies for SMTP Ports
- Accept & Deliver: spam mails are flagged and delivered to the recipient for further processing.
- Reject & Deliver: a configurable rejection message is returned to the sender and the spam mail is flagged and delivered to the recipient.
- Reject & Redirect: a configurable rejection message is returned to the sender, and the spam mail is redirected to a configurable mail account.
- Reject & Delete: a configurable rejection message is returned to the sender, the spam mail is deleted and not delivered to the recipient.
Mail Filtering Policies for POP3 Ports
- Deliver: spam mails are flagged and delivered to the recipient for further processing.
- Delete: spam mails are deleted.
Built-in POP3 to SMTP connector
A POP3 connector is a POP3 to SMTP protocol adapter. CleanMail's built-in POP3 Connector can be used to retrieve messages from a POP3 server, and forward these messages (after filtering) to a mail account on your SMTP server.
The forwarding process implements a safe transaction scheme: Only mails completely transmitted and accepted by the SMTP mail server are deleted from the POP3 mailbox. Mails not forwarded (ie. spam messsages, if you choose to delete spam messages) are just deleted and never enter your mail server.
Full Multi-Domain Support
Usually, CleanMail is a proxy only for a single mail server. Some users have more than one mail server and may want to filter mail for many servers and domains. CleanMail allows to configure an unlimited number of SMTP/POP3 proxy ports (or POP3 connectors). Each proxy has its own incoming and outgoing server settings and each has its own filter pipeline. You can use this to configure each proxy separately and adapt it to your needs.