Simple Mail Forwarder (SMF)

Simplest and Smallest Email Forward Service for Docker.

1. Config by one-line
2. Run as docker start
3. Image Size 10MB

View on GitHub | View on Docker

Voice from Users

@Brian Christner : After testing a ton of different mail projects I finally discovered Simple Mail Forwarder (SMF) by Huan. This image is based on Alpine which is already a positive. It is super tiny and as the name suggests, easy to use. link

@kachkaev : really happy to discover it! link

@kiani: have a working mail server, seriously, it was that easy. link

@counterbeing: great image. Wonderfully easy interface, with all that i need. 👍 link

@nelfer: Guess what? Your image already supports this! link

@Czocher: It's at least a timesaver if not a lifesaver. link

@StrangePeanut: I am so glad to have found SMF. Many thanks for this brilliant solution to email forwarding. link

What is Simple Mail Forwarder (SMF)?

If you have a domain name, only want to have one (or a few) email address(es) on this domain, and you want to forward all the emails to another email account - Simple Mail Forwarder (SMF) is exactly what you need. (with Docker)

Because of the above, this docker image was built for ultimate simplicity. I've owned many domains and needed email addresses for them (for fun and/or work). I hated configuring email servers. Some DNS providers provide free email forwarding services for their own domains, some do not. And almost all email forwarding services are NOT free. So I decided to make one myself (thanks docker).

Related Services

• DuoCircle Email Forwarding From $59.95/year
• Cloud Mail In From $9/month. And it is not for humans.
• MailGun professional service. Free plan includes 10,000 emails/month. but can result in your domain being treated as spam

I was willing to pay $10/year, but the cheapest plan I could find was $9 per month. Having a $10 USD machine with unlimited e-mail & domains per month is an amazing idea! And of course you could also put other dockers on this machine. :-D

Quick-start (TL;DR)

Just set SMF_CONFIG and run:

export SMF_CONFIG='testi@testo.com:test@test.com:test'
docker run -e SMF_CONFIG -p 25:25 zixia/simple-mail-forwarder

Don't forget to modify the DNS MX record of your domain. (in this example, it's testo.com)

This will forward all emails received by testi@testo.com to test@test.com.

If you want to forward all emails sent to domain testo.com to all@test.com, set it like so:

export SMF_CONFIG='@testo.com:all@test.com'

See? There is nothing easier.

If you want to run it constanly in the background add -t -d --restart=always after run:

docker run -t -d --restart=always -e SMF_CONFIG -p 25:25 zixia/simple-mail-forwarder

• -t: Allocate a pseudo-tty
• -d: Detached Mode
• --restart=always: Restart this container automatically

Otherwise, docker thinks that your applications stops and shutdown the container.

Quick Test

Tested by BATS(Bash Automated Testing System), a bash implementation of TAP(Test Anything Protol).

How to run:

$ docker run zixia/simple-mail-forwarder test
>> exec bats test
1..20
ok 1 confirm hostname pretend to work.
ok 2 confirm hwclock pretend to work.
ok 3 service postfix could start/stop right.
ok 4 SMF_CONFIG exist
ok 5 SMF_DOMAIN exist
ok 6 virtual maping source is set
ok 7 virtual maping data is set
ok 8 virtual maping db is set
ok 9 system hostname FQDN resolvable
ok 10 postfix myhostname FQDN & resolvable
ok 11 check other hostname setting
ok 12 confirm postfix is running
ok 13 confirm port 25 is open
ok 14 crond is running
ok 15 ESMTP STATTLS supported
ok 16 ESMTP AUTH supported
ok 17 ESMTP STARTTLS supported
ok 18 create user testi@testo.com by password test
ok 19 ESMTP AUTH by testi@testo.com/test
ok 20 ESMTP TLS AUTH by testi@testo.com/test

You are all set! :-]

Environment Variable and Default Values

SMF_CONFIG: MUST be defined. there's no default setting. (set me! I'm the only parameter you need to set~)

TZ : (Optional) set the timezone , IE EST5EDT or Europe/Rome

SMF_CONFIG Examples

Here's how to config the only required SMF_CONFIG environment parameter of SMF Docker:

1. Basic

Forward all emails received by testi@testo.com to test@test.com:

export SMF_CONFIG='testi@testo.com:test@test.com'

Forward all emails received by any email address in domain testo.com to all@test.com:

export SMF_CONFIG='@testo.com:all@test.com'

You could get the ESMTP AUTH password for you on your docker log. It's randomly generated if you do not provide one.

2. Advanced

Add ESMTP AUTH password:

export SMF_CONFIG='from@testi.com:to@testo.com:ThisIsPassword'

Password will be printed on the docker log.

3. Hardcore

Add as many email accounts as you want, with or without password. Seperated by semicolon or a new line:

export SMF_CONFIG='testi@testo.com:test@test.com:ThisIsPassword;testo@testi.com:test@test.com:AnotherPassword'

Tips: if you only provide the first password and leave the rest blank, then the passwords for all the rest accounts will be the same as the last password value you set. This is by design.

You can also forward all emails received by testi@testo.com to multiple destination addresses:

export SMF_CONFIG='testi@testo.com:test1@test.com|test2@test.com|test3@test.com'

SMF_RELAYHOST Examples

Here's how to configure a relayhost/smarthost to use for forwarding mail.

Send all outgoing mail trough a smarthost on 192.168.1.2

export SMF_RELAYHOST='192.168.1.2'

SMF_RELAYAUTH Examples

If the SMF_RELAYHOST require authentication,

export SMF_RELAYAUTH='username@relayhost.com:RelayHostPassword'

TLS (SSL) Certificates

SMF creates its own certificate and private key when it starts. This certificate is self signed, so some systems might give you a warning about the server not being trusted. If you have valid certificates for the domain name of the host, then you can use them and avoid the warning about not being trusted.

1. First you need to prepare the certificate files. Copy your full chain certificate to a file named smtp.cert (or smtp.ec.cert if it contains a EC certificate). Then copy the private key to a file named smtp.key (or smtp.ec.key if it contains a EC key)

2. Copy these files to a folder. For example: /data/certs/. This folder will be mounted as a volume in SMF

3. When creating the container, add the -v (volume) parameter to mount it to the folder /etc/postfix/cert/ like so:

   docker run  -e SMF_CONFIG -p 25:25 -v /data/certs/:/etc/postfix/cert/ zixia/simple-mail-forwarder

4. Your emails should now be forwarded with trusted encryption. You can use this tool to test it: http://checktls.com/

If you do not have a certificate and don't have the budget to afford one, you can use https://letsencrypt.org if you have shell access to the server (Note, SMF does not provide this service, yet). Letsencrypt allows you to create valid trusted certificates for a server, if the server responds to the domain you specify. In order to do this, you need to run the program from within the server and have administrator rights.

1. First install letsencrypt. This might vary by distribution, but in Ubuntu it is like this:

   sudo apt-get install letsencrypt

2. Stop any web server that might be using port 80 (Apache, nginx, etc)

3. Determine all of the domains and subdomains that you want the certificate to cover, for example mydomain.com, www.mydomain.com, smtp.mydomain.com, etc. Remember to include the domain that SMF will respond to (as per MX record in DNS configuration of the domain)

4. Execute the following command (you can add as many domains as you wish with the -d option. But remember, their DNS resolution must resolve to the server where letsencrypt is being executed)

   letsencrypt certonly --standalone -d yourdomain.com -d www.yourdomain.com -d mail.yourdomain.com

5. Follow the prompts and if everything is successful you will get your certificates in a folder like /etc/letsencrypt/live/mydomain.com

6. You can now use those certificates to make SMF TLS trusted.

This was a quick way of how to use letsencrypt. For a full tutorial based on your OS see: https://certbot.eff.org/

DKIM

SMF will generate private/public keypairs for $SMF_DOMAIN and for all source domains contained in SMF_CONFIG. All keys will be stored in /var/db/dkim/<domain.tld>/.

This will enable DKIM for multiple domains and test for their validity on SMF startup.

Public key must be set as TXT record in DNS under default._domainkey name. default._domainkey can be found in /var/db/dkim/<domain.tld>/default.txt.

It is highly advised to mount /var/db/dkim/ folder to host, so generated keypair would not get lost/regenerated:

docker run -e SMF_CONFIG -p 25:25 -v $(pwd)/dkim:/var/db/dkim/ zixia/simple-mail-forwarder

SMF allows using environment variables to change the size of the DKIM key:

• SMF_DKIM_KEYSIZE will set the keysize of the generated DKIM key (setting opendkim-genkey -b $SMF_DKIM_KEYSIZE). Defaults to 2048.

Stripping sender details

SMF will strip the sender's IP, client, and user agent headers when the SMF_SENDERPRIVACY environment variable is defined.

Custom postfix configuration

SMF allows using environment variables to add or change lines to postfix main.cf and master.cf configuration files.

• SMF_POSTFIXMAIN_* variables will edit postfix main.cf entries.

  • Format: SMF_POSTFIXMAIN_option_name=value
  • Example: SMF_POSTFIXMAIN_soft_bounce=yes will call postconf -e soft_bounce=yes and add the option.

• SMF_POSTFIXMASTER_* variables will edit postfix master.cf.

  • Format: SMF_POSTFIXMASTER_service__name__type__parameter_name=value Please note the double underscore __ to differentiate service and type (which require a / as separation symbol) and the single underscore _ used only by parameter names.
  • Example: SMF_POSTFIXMASTER_submission__inet__smtpd__recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject will call postconf -P submission/inet/smtpd/recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject and add the option.

Logging

The default log configuration of postfix is sending all log entries to /dev/stdout. Because file logs are important in some cases, SMF allows overriding the default log configuration of postfix.

• SMF_POSTFIXLOG will edit postfix logging configuration
  • Format: SMF_POSTFIXLOG=value
  • Example: SMF_POSTFIXLOG=/var/log/postfix.log will call postconf maillog_file="/var/log/postfix.log"

Attention: The logfile path must start with "/var".

When you wish to rotate logs, look at the postfix logrotate command in the official documentation.

Enable SRS (Sender Rewriting Scheme)

SRS is required if the sender uses SPF for verification. If SRS is not enabled, SPF verification will fail. (Additional Informations to SRS)

You can enable SRS by setting the environment variable SMF_SRS=true. This will start PostSRSd inside the container. The secret is automatically created and the domain is set to SMF_DOMAIN. Now you just need to set the following environment variables for Postfix:

SMF_POSTFIXMAIN_sender_canonical_maps=tcp:localhost:10001
SMF_POSTFIXMAIN_sender_canonical_classes=envelope_sender
SMF_POSTFIXMAIN_recipient_canonical_maps=tcp:localhost:10002
SMF_POSTFIXMAIN_recipient_canonical_classes=envelope_recipient,header_recipient

Helper Scripts

1. Build from source.

   ./script/build.sh latest

2. Run a self-test for SMF docker.

   ./script/run.sh latest test

3. Get a shell inside SMF docker.

   ./script/devshell.sh latest

Manual Test

$ telnet 127.0.0.1 25
> 220 testo.com ESMTP
ehlo test.com
> 250-testo.com
> 250-STARTTLS
> 250-AUTH PLAIN LOGIN
auth plain
> 334
dGVzdGlAdGVzdG8uY29tAHRlc3RpQHRlc3RvLmNvbQB0ZXN0
> 235 2.7.0 Authentication successful
quit
> 221 2.0.0 Bye
> Connection closed by foreign host

P.S. The magic string dGVzdGlAdGVzdG8uY29tAHRlc3RpQHRlc3RvLmNvbQB0ZXN0 stands for testi@testo.com\0test@testo.com\0test in base64 encoding, required by AUTH PLAIN.

Useful article about SMTP Authentication: http://www.fehcom.de/qmail/smtpauth.html

Bug

Github Issue - https://github.com/huan/docker-simple-mail-forwarder/issues

Useful Articles

1. How to use docker to forward emails sent to your domain to your email (works for Gmail)

Changelog

master

v1.4 (Mar 9, 2021)

1. Update DockerHub README.md @thexperiments #94
2. Allow for setting any Postfix variables in the config file (both main.cf and master.cf) @dgraziotin #93
3. Strips sender details (IP, client, user agent) when sending @dgraziotin #91
4. Adds DKIM support for multiple domains @dgraziotin #89
5. DKIM support for multiple domains @dgraziotin #88
6. Add DKIM support @petslane #83
7. Use script to install s6 with right platforms (arm/x86) @huan #76

v1.3 (17 Sep 2020)

1. Upgrade alpine docker base image to 3.12
2. Fix timeout args breaking changes
3. Fix etc and libexec path change for postfix

v1.1 (09 Jun 2019)

1. Timezone support by @me1299 #56 #57
2. EC key support by @Czocher #51
3. Upgrade CircleCI from v1 to v2 @huan
4. Fix typo in README @universeroc #47
5. Update Base to Alpine 3.8 @riptidewave93 #43
6. Update alpine:latest as base image @martijnrondeel #37

v1.0.0 (25 Jul 2017)

• Release v1.0

v0.4.3 (14 Jul 2017)

1. Add a note about running it in the background to prevent docker auto shutdown. by @delmicio #27
2. Added smarthost support by @Duumke #22
3. Added support for mynetworks by @SamMousa #20
4. Allow own certificates by @nelfer #15
5. Updated documentation for forward all emails @nelfer #14
6. ARM version of armhf by @dimitrovs #12
7. use SMF_DOMAIN env for certificate's CN by @bcardiff #11
8. allow multiple forwards separated by | by @kminek #7
9. Update docker-compose.yml to fix tutum tag by @vegasbrianc #4

v0.4.2 (25 Sep 2016)

1. close issue #1
2. increace message size limit from 10MB to 40MB
3. fix domain name in scripts
4. fix unit test fail error: do not upgrade alpine
5. restore deploy button in readme: it is docker cloud now.(former tutum)

v0.4.0

1. switch FROM image from alpine to sillelien/base-alpine
2. manage postfix service by S6
3. solve PID 1 Zombie Problem
4. enhanced busybox shell
5. NOT to use OpenRC(very buggy run inside docker container) any more!
6. better ESMTP TLS AUTH test script
7. docker image size: 10MB

v0.3.0 <- Don't use me, I'm BUGGY

1. CI(continuous integration) supported by use CircleCI
2. CD(continuous delivery) supported by use Tutum Button
3. write better tests
4. tune OpenRC inside alpine linux
5. full description README
6. docker image size: 7MB

v0.2.0 <- Don't use me, I'm BUGGY

1. supported specify user password
2. supported ESMTP TLS
3. docker image size: 7MB

v0.1.0

1. dockerized
2. basic forward function
3. self-testing
4. docker image size: 6MB

Cloud Requirement

• A Cloud Service that could host docker is required.

  • DigitalOcean.com get $10 free credit(cover 2 months vpsv cost) by register from here.
  • LiNode.com
  • Netdedi SSD VPS and Dedicated Server with NetDedi(Koera)

• A Docker management platform is recommanded.

  • Docker Cloud(Former Tutum)

• Docker is required.

  • Docker.com

CONTRIBUTORS

Author

• Huan LI (李卓桓) zixia@zixia.net

MAINTAINERS

• Stefan Dimitrov @dimitrovs

COPYRIGHT & LICENSE

• Code & Docs © 2015 - now Huan LI zixia@zixia.net
• Code released under the Apache-2.0 License
• Docs released under Creative Commons