emailrelay/doc/userguide.md

E-MailRelay User Guide

What is it?

E-MailRelay is an e-mail store-and-forward message transfer agent and proxy server. It runs on Unix-like operating systems (including Linux and Mac OS X), and on Windows.

E-MailRelay does three things: it stores any incoming e-mail messages that it receives, it forwards e-mail messages on to another remote e-mail server, and it serves up stored e-mail messages to local e-mail reader programs. More technically, it acts as a SMTP storage daemon, a SMTP forwarding agent, and a POP3 server.

Whenever an e-mail message is received it can be passed through a user-defined program, such as a spam filter, which can drop, re-address or edit messages as they pass through.

E-MailRelay uses the same non-blocking i/o model as Squid and nginx giving excellent scalability and resource usage.

C++ source code is available and distribution is permitted under the GNU General Public License V3.

What it's not

E-MailRelay does not normally do routing of individual messages; it is not a routing MTA. It forwards all e-mail messages to a pre-configured SMTP server, regardless of any message addressing or DNS redirects.

Why use it?

E-MailRelay is a simple tool that does SMTP. For simple tasks it is likely to be easier to understand and configure than a more general purpose MTA.

Typical applications of E-MailRelay include:

• spam filtering and virus checking incoming mail
• adding digital signatures or legal disclaimers to outgoing mail
• store-and-forward for outgoing mail across an intermittent internet connection
• adding authentication and encryption where the existing infrastructure does not support it
• taking messages in with SMTP and serving them to local POP clients
• giving multiple POP clients independent copies of incoming e-mail
• SMTP proxying by running as a proxy server on a firewall machine

The code has few dependencies on third-party libraries or run-time environments so it is easy to build and install.

E-MailRelay is designed to be policy-free, so that you can implement your own policies for message retries, bounces, local mailbox delivery, spam filtering etc. through external scripts.

Running E-MailRelay

To use E-MailRelay in store-and-forward mode use the --as-server option to start the storage daemon in the background, and then do delivery of spooled messages by running with the --as-client option.

For example, to start a storage daemon listening on port 587 use a command like this:

    emailrelay --as-server --port 587 --spool-dir /tmp

And then to forward the spooled mail to smtp.example.com run something like this:

    emailrelay --as-client smtp.example.com:25 --spool-dir /tmp

To get behaviour more like a proxy you can add the --poll and --forward-to options so that messages are forwarded continuously rather than on-demand.

This example starts a store-and-forward server that forwards spooled-up e-mail every minute:

    emailrelay --as-server --poll 60 --forward-to smtp.example.com:25

Or for a proxy server that forwards each message soon after it has been received, you can use --as-proxy or add --forward-on-disconnect:

    emailrelay --as-server --forward-on-disconnect --forward-to smtp.example.com:25

To edit or filter e-mail as it passes through the proxy specify your filter program with the --filter option, something like this:

    emailrelay --as-proxy smtp.example.com:25 --filter addsig.js

To run E-MailRelay as a POP server without SMTP use --pop and --no-smtp:

    emailrelay --pop --no-smtp --log --close-stderr

The emailrelay-submit utility can be used to put messages straight into the spool directory so that the POP clients can fetch them.

By default E-MailRelay will always reject connections from remote networks. To allow connections from anywhere use the --remote-clients option, but please check your firewall settings to make sure this cannot be exploited by spammers.

For more information on the command-line options refer to the reference guide or run:

    emailrelay --help --verbose

Configuration

The emailrelay program itself is mostly configured through command-line options (such as --port and --forward-to).

In most installations on Unix-like system the E-MailRelay server will be started up by the boot-time script called emailrelay in the /etc/init.d directory, and this script uses the configuration file /etc/emailrelay.conf to define the server command-line. Each entry in the configuration file corresponds to an E-MailRelay command-line option, so you can edit this file to add and remove server options. Refer to the reference guide for a complete list of configuration options.

On Windows the installation program creates a startup batch file called emailrelay-start.bat that contains all the server command-line options and you can edit this file to change the server configuration. You can also set up your own shortcuts to the E-MailRelay executable and add and remove command-line options using the shortcut properties tab.

If you are using authentication then you will have to create the text files containing your authentication secrets (account names, passwords and password hashes). The --server-auth, --client-auth and --pop-auth command-line options are used to point to these files.

There is also a graphical configuration program called emailrelay-gui that may be available to help with configuring the system. This is mostly intended to be used once at installation time since it is the basis of the Windows installer, but it can also be used to do some simple reconfiguration of an already-installed system. It takes you through a sequence of configuration pages and then on the last page creates or updates the configuration files, ie. the authentication secrets file and the configuration file emailrelay.conf or emailrelay-start.bat.

Logging

If the --log option is used then E-MailRelay program sends warnings and error messages to its standard error stream, and to the syslog system on Unix or to the Event Viewer on Windows.

The standard error stream logging can be redirected to a file by using the --log-file option, and daily log files can be created by using %d in the filename.

For more verbose logging add the --verbose option to the command-line.

Preventing open mail relay

If you are running E-MailRelay as a server with a permanent connection to the internet it is important to prevent open mail relay because this can be exploited by spammers and get you into trouble with your ISP. By default open mail relaying is not possible because E-MailRelay does not accept IP connections from remote clients. However, if you use the --remote-clients option then you need to be more careful.

If the only required access to the E-MailRelay server is from a local network and not from the internet then just set up your firewall to block incoming connections on ports 25 (SMTP) and 110 (POP) and also use the --interface option on the E-MailRelay command-line so that it only listens for incoming connections on the local network.

If you do need to accept connections from the internet then you should require all clients to authenticate themselves by using the --server-auth option on the E-MailRelay command-line. If you also want local clients running on your internal network to be able to bypass this authentication then you can put those trusted IP addresses in the E-MailRelay secrets file with an authentication mechanism of none. Refer to the reference guide for more information.

Running as a POP server

E-MailRelay can run as a POP server so that e-mail client programs can retrieve messages from the E-MailRelay spool directory directly.

To allow POP access to spooled messages use a command-line something like this:

    emailrelay --as-server --pop --pop-auth=/etc/pop.auth

You will need to create the authentication secrets file (pop.auth in this example) containing usernames and passwords. A simple example would look like this:

    server plain user1 password1
    server plain user2 password2

It can sometimes be useful to serve up the same e-mail messages to multiple POP clients. For example, you might use several e-mail clients on your local network and you would like to see your mail on all of them equally. The --pop-by-name option is intended to be used in this scenario; each e-mail client sees its own copy of the e-mail messages, stored in its own sub-directory of the main spool directory. The name of the sub-directory is simply the name that the client uses to authenticate with the E-MailRelay server. You just need to create the sub-directory for each client and then specify emailrelay-filter-copy as the E-MailRelay --filter program.

Refer to the documentation of the various --pop command-line options for more detail: --pop, --pop-port, --pop-auth, --pop-no-delete and --pop-by-name.

Triggering delivery

If you are using E-MailRelay on Unix to store and forward e-mail over an intermittent link to the internet such as dial-up or wireless networking, then you might need to set things up so that the network tells E-MailRelay when to start forwarding e-mail.

On Linux systems you should find that there are special directories where you can install your own hook scripts that are called whenever a dial-up or wireless network connection is established. For dial-up this might be /etc/ppp/ip-up.d, and for wireless /etc/network/if-up.d.

Just create a two-line script like this in the relevant directory:

    #!/bin/sh
    exec /usr/local/sbin/emailrelay --as-client=smtp.example.com:smtp

and make it executable using chmod +x.

Failed e-mails

If e-mail messages cannot be forwarded by the E-MailRelay system then the envelope files in the spool directory are given a .bad suffix. The reason for the failure will be recorded in the envelope file itself.

You should check for .bad envelope files in the E-MailRelay spool directory from time to time. If you want them to be retried next time then just remove the .bad filename suffix.

You can run the emailrelay-resubmit script periodically to automate this; it removes the .bad suffix from files in the spool directory as long as they have not been retried too many times already.

Usage patterns

The simplest ways of using E-MailRelay for SMTP are to run it as a proxy or to do store-and-forward, but many other configurations are possible. For example, multiple E-MailRelay servers can run in parallel sharing the same spool directory, or they can be chained in series to that e-mail messages get transferred from one to the next.

Remember that messages can be introduced directly into the E-MailRelay spool directory using the emailrelay-submit utility, and they can be moved out again at any time, as long as the envelope file is not locked (ie. with a special filename extension). Your --filter program can edit messages in any way you want, and it can even delete the current message from the spool directory.

When using E-MailRelay as a POP server the --pop-by-name feature can be used to serve up different spooled messages according to the username that the client authenticated with. Rather than use emailrelay-filter-copy to distribute incoming e-mail messages into all subdirectories you could use a custom script to do it based on the message addressing.

The POP server can also be used for checking e-mails that are taken out of the normal store-and-forward flow. For example, a --filter script that checks for spam could move suspicious e-mails into a subdirectory of the spool directory that is accessible via the --pop-by-name feature.

Rate limiting

If you need to slow the rate at which e-mails are forwarded you can use a --client-filter program to introduce a delay. On Windows this JavaScript program would give you a delay of a minute:

    WScript.Sleep( 60000 ) ;
    WScript.Quit( 0 ) ;

However, this can cause timeouts at the server, so a better approach is to use --client-filter exit:102 so that only one e-mail message is forwarded on each polling cycle, and then use --poll 60 to limit it to one e-mail per minute.

SpamAssassin

The E-MailRelay server can use SpamAssassin to mark or reject potential spam.

It's easiest to run SpamAssassin's spamd program in the background and let E-MailRelay send incoming messages to it over the network. By default SpamAssassin spamd uses port 783 so you should use an E-MailRelay command-line option of --filter spam-edit:127.0.0.1:783 if you want spam messages to pass through with a warning added, or --filter spam:127.0.0.1:783 if you want spam messages to be rejected outright.

Alternatively you can run SpamAssassin on demand for each message. To get E-MailRelay to reject spam outright you can just use spamassassin -e as your E-MailRelay --filter program:

    emailrelay --as-server --filter="/usr/bin/spamassassin -e"

Or on Windows:

    emailrelay --as-server --filter="c:/perl/site/bin/spamassassin.bat -e"

To get spam messages identified by SpamAssassin but still pass through the E-MailRelay system you will have to have a small --filter script to collect the output from the spamassassin program and write it back into the E-MailRelay content file.

On Unix your --filter shell script could look something like this:

    #!/bin/sh
    spamassassin "$1" > "$1.tmp"
    mv "$1.tmp" "$1"
    exit 0

On Windows an equivalent batch script would be:

    c:\perl\site\bin\spamassassin.bat %1 > %1.tmp
    ren %1.tmp %1
    exit 0

Google mail

To send mail via Google mail's SMTP gateway you will need to obtain a new application password from Google. Log in to your Google account and look for the account's security settings and then app passwords. Create the password for E-MailRelay as application type other.

Then you need to create a client secrets file for E-MailRelay containing your account name and the new application password. You may already have this file on Windows as C:\ProgramData\E-MailRelay\emailrelay.auth.

You should edit the file to contain one client line, something like this:

    client plain myname@gmail.com myapppassword

Then change the E-MailRelay startup batch file or configuration file to refer to your secrets file by using the --client-auth option. The --as-proxy or --forward-to options should be set to smtp.gmail.com:587 and you will also need to add the --client-tls option to enable TLS encryption.

On Windows the E-MailRelay startup batch file should contain something like this:

    emailrelay --as-proxy=smtp.gmail.com:587 --client-tls --client-auth=C:/ProgramData/E-MailRelay/emailrelay.auth ...

Or in a configration file like this:

    forward-to smtp.gmail.com:587
    client-tls
    client-auth C:/ProgramData/E-MailRelay/emailrelay.auth

Connection tunnelling

E-MailRelay can send mail out via a SOCKS v4 proxy, which makes it easy to route your mail through an encrypted tunnel created by ssh -N -D or via the Tor anonymising network.

For example, this will run an E-MailRelay proxy on port 587 that routes via a local Tor server on port 9050 to the mail server at smtp.example.com:

    emailrelay --port 587 --as-proxy=smtp.example.com:25@localhost:9050 --domain=anonymous.net --anonymous --connection-timeout=300

Blocklists and dynamic firewalls

E-MailRelay can consult with remote DNSBL blocklist servers in order to block incoming connections from known spammers. Refer to the documentation of the --dnsbl option for more details.

It is also possible to integrate E-MailRelay with intrusion detection systems such as fail2ban that monitor log files and dynamically modify your iptables firewall. Use E-MailRelay's --log-address command-line option so that the remote IP address of any badly-behaved remote user is logged and made available to fail2ban.

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Copyright (C) 2001-2022 Graeme Walker