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bokysan / docker-postfix

Multi architecture simple SMTP server (postfix null relay) host for your Docker and Kubernetes containers. Based on Debian/Ubuntu/Alpine.
MIT License
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dkim dkim-keys docker docker-compose docker-container docker-image docker-postfix helm-chart kubernetes mail mailserver postfix postfix-configuration postfix-docker postfix-relay postfix-server relaying-messages smtp smtp-server

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docker-postfix

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Simple postfix relay host ("postfix null client") for your Docker containers. Based on Debian (default), Ubuntu and Alpine Linux. Feel free to pick your favourite distro.

Table of contents

Description

This image allows you to run POSTFIX internally inside your docker cloud/swarm installation to centralise outgoing email sending. The embedded postfix enables you to either send messages directly or relay them to your company's main server.

This is a server side POSTFIX image, geared towards emails that need to be sent from your applications. That's why this postfix configuration does not support username / password login or similar client-side security features.

IF YOU WANT TO SET UP AND MANAGE A POSTFIX INSTALLATION FOR END USERS, THIS IMAGE IS NOT FOR YOU. If you need it to manage your application's outgoing queue, read on.

TL;DR

To run the container, do the following:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com" -p 1587:587 boky/postfix

or

helm repo add bokysan https://bokysan.github.io/docker-postfix/
helm upgrade --install --set persistence.enabled=false --set config.general.ALLOW_EMPTY_SENDER_DOMAINS=yes mail bokysan/mail

You can also find this image at ArtifactHub.

You can now send emails by using localhost:1587 (on Docker) as your SMTP server address. Note that if you haven't configured your domain to allow sending from this IP/server/nameblock, your emails will most likely be regarded as spam.

All standard caveats of configuring the SMTP server apply:

If you don't know what any of the above means, get some help. Google is your friend. It's also worth noting that it's pretty difficult to host a SMTP server on a dynamic IP address.

Please note that the image uses the submission (587) port by default. Port 25 is not exposed on purpose, as it's regularly blocked by ISPs, already occupied by other services, and in general should only be used for server-to-server communication.

Updates

v4.0.0

Several potentially "surprising" changes went into this issue and hence warrant a version upgrade:

v3.0.0

There's a potentially breaking change introduced now in v3.0.0: Oracle has changed the license of BerkleyDB to AGPL-3.0, making it unsuitable to link to packages with GPL-incompatible licenses. As a result Alpine (on which this image is based) has deprecated BerkleyDB throughout the image:

Support for Postfix hash and btree databases has been removed. lmdb is the recommended replacement. Before upgrading, all tables in /etc/postfix/main.cf using hash and btree must be changed to a supported alternative. See the Postfix lookup table documentation for more information.

While this should not affect most of the users (/etc/postfix/main.cf is managed by this image), there might be use cases where people have their own configuration which relies on hash and btree databases. To avoid braking live systems, the version of this image has been updated to v3.0.0.

Architectures

Available for all your favourite architectures. Run in your server cluster. Run it on your Raspberry Pi 4. Run it on your ancient Pentium or an old Beaglebone. The following architectures are supported: linux/386, linux/amd64, linux/arm/v6, linux/arm/v7, linux/arm64 and linux/ppc64le.

Configuration options

General options

Inbound debugging

Enable additional debugging for any connection coming from POSTFIX_mynetworks. Set to a non-empty string (usually 1 or yes) to enable debugging.

ALLOWED_SENDER_DOMAINS and ALLOW_EMPTY_SENDER_DOMAINS

Due to in-built spam protection in Postfix you will need to specify sender domains -- the domains you are using to send your emails from, otherwise Postfix will refuse to start.

Example:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -p 1587:587 boky/postfix

If you want to set the restrictions on the recipient and not on the sender (anyone can send mails but just to a single domain for instance), set ALLOW_EMPTY_SENDER_DOMAINS to a non-empty value (e.g. true) and ALLOWED_SENDER_DOMAINS to an empty string. Then extend this image through custom scripts to configure Postfix further.

Log format

The image will by default output logs in human-readable (plain) format. If you are deploying the image to Kubernetes, it might be worth changing the output format to json as it's more easily parsable by tools such as Prometheus.

To change the log format, set the (unsurprisingly named) variable LOG_FORMAT=json.

Postfix-specific options

RELAYHOST, RELAYHOST_USERNAME and RELAYHOST_PASSWORD

Postfix will try to deliver emails directly to the target server. If you are behind a firewall, or inside a corporation you will most likely have a dedicated outgoing mail server. By setting this option, you will instruct postfix to relay (hence the name) all incoming emails to the target server for actual delivery.

Example:

docker run --rm --name postfix -e RELAYHOST=192.168.115.215 -p 1587:587 boky/postfix

You may optionally specifiy a relay port, e.g.:

docker run --rm --name postfix -e RELAYHOST=192.168.115.215:587 -p 1587:587 boky/postfix

Or an IPv6 address, e.g.:

docker run --rm --name postfix -e 'RELAYHOST=[2001:db8::1]:587' -p 1587:587 boky/postfix

If your end server requires you to authenticate with username/password, add them also:

docker run --rm --name postfix -e RELAYHOST=mail.google.com -e RELAYHOST_USERNAME=hello@gmail.com -e RELAYHOST_PASSWORD=world -p 1587:587 boky/postfix

POSTFIX_smtp_tls_security_level

Define relay host TLS connection level. See smtp_tls_security_level for details. By default, the permissive level ("may") is used, which basically means "use TLS if available" and should be a sane default in most cases.

This level defines how the postfix will connect to your upstream server.

XOAUTH2_CLIENT_ID, XOAUTH2_SECRET, XOAUTH2_INITIAL_ACCESS_TOKEN, XOAUTH2_INITIAL_REFRESH_TOKEN and XOAUTH2_TOKEN_ENDPOINT

Note: These parameters are used when RELAYHOST and RELAYHOST_USERNAME are provided.

These parameters allow you to configure a relayhost that requires (or recommends) the XOAuth2 authentication method (e.g. GMail).

Example:

docker run --rm --name pruebas-postfix \
    -e RELAYHOST="[smtp.gmail.com]:587" \
    -e RELAYHOST_USERNAME="<put.your.account>@gmail.com" \
    -e POSTFIX_smtp_tls_security_level="encrypt" \
    -e XOAUTH2_CLIENT_ID="<put_your_oauth2_client_id>" \
    -e XOAUTH2_SECRET="<put_your_oauth2_secret>" \
    -e ALLOW_EMPTY_SENDER_DOMAINS="true" \
    -e XOAUTH2_INITIAL_ACCESS_TOKEN="<put_your_acess_token>" \
    -e XOAUTH2_INITIAL_REFRESH_TOKEN="<put_your_refresh_token>" \
    boky/postfix

Next sections describe how to obtain these values.

OAuth2 Client Credentials (GMail)

Visit the Google API Console to obtain OAuth 2 credentials (a client ID and client secret) for an "Installed application" application type.

Save the client ID and secret and use them to initialize XOAUTH2_CLIENT_ID and XOAUTH2_SECRET respectively.

We'll also need these credentials in the next step.

Obtain Initial Access Token (GMail)

Use the Gmail OAuth2 developer tools to obtain an OAuth token by following the Creating and Authorizing an OAuth Token instructions.

Save the resulting tokens and use them to initialize XOAUTH2_INITIAL_ACCESS_TOKEN and XOAUTH2_INITIAL_REFRESH_TOKEN.

Debug XOAuth2 issues

If you have XOAuth2 authentication issues you can enable XOAuth2 debug message setting XOAUTH2_SYSLOG_ON_FAILURE to "yes" (default: "no"). If you need a more detailed log trace about XOAuth2 you can set XOAUTH2_FULL_TRACE to "yes" (default: "no").

MASQUERADED_DOMAINS

If you don't want outbound mails to expose hostnames, you can use this variable to enable Postfix's address masquerading. This can be used to do things like rewrite lorem@ipsum.example.com to lorem@example.com.

Example:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -e "MASQUERADED_DOMAINS=example.com" -p 1587:587 boky/postfix

SMTP_HEADER_CHECKS

This image allows you to execute Postfix header checks. Header checks allow you to execute a certain action when a certain MIME header is found. For example, header checks can be used prevent attaching executable files to emails.

Header checks work by comparing each message header line to a pre-configured list of patterns. When a match is found the corresponding action is executed. The default patterns that come with this image can be found in the smtp_header_checks file. Feel free to override this file in any derived images or, alternately, provide your own in another directory.

Set SMTP_HEADER_CHECKS to type and location of the file to enable this feature. The sample file is uploaded into /etc/postfix/smtp_header_checks in the image. As a convenience, setting SMTP_HEADER_CHECKS=1 will set this to regexp:/etc/postfix/smtp_header_checks.

Example:

docker run --rm --name postfix -e "SMTP_HEADER_CHECKS="regexp:/etc/postfix/smtp_header_checks" -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -p 1587:587 boky/postfix

POSTFIX_myhostname

You may configure a specific hostname that the SMTP server will use to identify itself. If you don't do it, the default Docker host name will be used. A lot of times, this will be just the container id (e.g. f73792d540a5) which may make it difficult to track your emails in the log files. If you care about tracking at all, I suggest you set this variable, e.g.:

docker run --rm --name postfix -e "POSTFIX_myhostname=postfix-docker" -p 1587:587 boky/postfix

POSTFIX_mynetworks

This implementation is meant for private installations -- so that when you configure your services using docker compose you can just plug it in. Precisely because of this reason and the prevent any issues with this postfix being inadvertently exposed on the internet and then used for sending spam, the default networks are reserved for private IPv4 IPs only.

Most likely you won't need to change this. However, if you need to support IPv6 or strenghten the access further, you can override this setting.

Example:

docker run --rm --name postfix -e "POSTFIX_mynetworks=10.1.2.0/24" -p 1587:587 boky/postfix

POSTFIX_message_size_limit

Define the maximum size of the message, in bytes. See more in Postfix documentation.

By default, this limit is set to 0 (zero), which means unlimited. Why would you want to set this? Well, this is especially useful in relation with RELAYHOST setting. If your relay host has a message limit (and usually it does), set it also here. This will help you "fail fast" -- your message will be rejected at the time of sending instead having it stuck in the outbound queue indefinitely.

Overriding specific postfix settings

Any Postfix configuration option can be overriden using POSTFIX_<name> environment variables, e.g. POSTFIX_allow_mail_to_commands=alias,forward,include. Specifying no content (empty variable) will remove that variable from postfix config.

SKIP_ROOT_SPOOL_CHOWN

Setting this to 1 will skip re-owning in /var/spool/postfix/ and /var/spool/postfix/pid. You generally do not want to set this option unless you're running into specific issues (e.g. #97).

If unsure, leave it as is.

ANONYMIZE_EMAILS

Anonymize email in Postfix logs. It mask the email content by putting * in the middle of the name and the domain. For example: from=<a*****************s@a***********.com>

Syntax: <masker-name>[?option=value&option=value&....]

NOTICE: Options are URL-encoded.

The following filters are provided with this implementation:

The default (smart) filter

Enable the filter by setting ANONYMIZE_EMAILS=smart.

The is enabled by setting the value to on, true, 1, default or smart. The filter will take an educated guess at how to best mask the emails, specifically:

E.g.:

Configuration parameters:

Property Default value Required Description
mask_symbol * no Mask symbol to use instead of replaced characters
The paranoid filter

The paranoid filter works similar to smart filter but will:

E.g.:

Configuration parameters:

Property Default value Required Description
mask_symbol * no Mask symbol to use instead of replaced characters
The hash filter

This filter will replace the email with the salted (HMAC - SHA256) hash. While it makes the logs much less readable, it has one specific benefit: it allows you to search through the logs if you know the email address you're looking for. You are able to calculate the hash yourself and then grep through the logs for this specific email address.

E.g.:

Filter will not work without configuration. You will need to provide (at least) the salt, e.g.: ANONYMIZE_EMAILS=hash?salt=demo

Configuration parameters:

Property Default value Required Description
salt none yes HMAC key (salt) used for calculating the checksum
prefix `` no Prefix of emails in the log (for easier grepping)
suffix `` no Suffix of emails in the log (for easier grepping)
split false no Set to 1, t or true to hash separately the local and the domain part
short_sha false no Set to 1, t or true to return just the first 8 characters of the hash
case_sensitive true no Set to 0, f or false to convert email to lowercase before hashing
The noop filter

This filter doesn't do anything. It's used for testing purposes only.

Writing your own filters

It's easy enough to write your own filters. The simplest way would be to take the email-anonymizer.py file in this image, write your own and then attach it to the container image under /scripts. If you're feeling adventureus, you can also install your own Python package -- the script will automatically pick up the class name.

DKIM / DomainKeys

This image is equipped with support for DKIM. If you want to use DKIM you will need to generate DKIM keys. These can be either generated automatically, or you can supply them yourself.

The DKIM supports the following options:

Supplying your own DKIM keys

If you want to use your own DKIM keys, you'll need to create a folder for every domain you want to send through. You will need to generate they key(s) with the opendkim-genkey command, e.g.

mkdir -p /host/keys; cd /host/keys

for DOMAIN in example.com example.org; do
    # Generate a key with selector "mail"
    opendkim-genkey -b 2048 -h rsa-sha256 -r -v --subdomains -s mail -d $DOMAIN
    # Fixes https://github.com/linode/docs/pull/620
    sed -i 's/h=rsa-sha256/h=sha256/' mail.txt
    # Move to proper file
    mv mail.private $DOMAIN.private
    mv mail.txt $DOMAIN.txt
done
...

opendkim-genkey is usually in your favourite distribution provided by installing opendkim-tools or opendkim-utils.

Add the created <domain>.txt files to your DNS records. Afterwards, just mount /etc/opendkim/keys into your image and DKIM will be used automatically, e.g.:

docker run --rm --name postfix -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -v /host/keys:/etc/opendkim/keys -p 1587:587 boky/postfix

Auto-generating the DKIM selectors through the image

If you set the environment variable DKIM_AUTOGENERATE to a non-empty value (e.g. true or 1) the image will automatically generate the keys.

Be careful when using this option. If you don't bind /etc/opendkim/keys to a persistent volume, you will get new keys every single time. You will need to take the generated public part of the key (the one in the .txt file) and copy it over to your DNS server manually.

Changing the DKIM selector

mail is the default DKIM selector and should be sufficient for most usages. If you wish to override the selector, set the environment variable DKIM_SELECTOR, e.g. ... -e DKIM_SELECTOR=postfix. Note that the same DKIM selector will be applied to all found domains. To override a selector for a specific domain use the syntax [<domain>=<selector>,...], e.g.:

DKIM_SELECTOR=foo,example.org=postfix,example.com=blah

This means:

Overriding specific OpenDKIM settings

Any OpenDKIM configuration option can be overriden using OPENDKIM_<name> environment variables, e.g. OPENDKIM_RequireSafeKeys=yes. Specifying no content (empty variable) will remove that variable from OpenDKIM config.

Verifying your DKIM setup

I strongly suggest using a service such as dkimvalidator to make sure your keys are set up properly and your DNS server is serving them with the correct records.

Docker Secrets / Kubernetes secrets

As an alternative to passing sensitive information via environment variables, _FILE may be appended to some environment variables (see below), causing the initialization script to load the values for those variables from files present in the container. In particular, this can be used to load passwords from Docker secrets stored in /run/secrets/<secret_name> files. For example:

docker run --rm --name pruebas-postfix \
    -e RELAYHOST="[smtp.gmail.com]:587" \
    -e RELAYHOST_USERNAME="<put.your.account>@gmail.com" \
    -e POSTFIX_smtp_tls_security_level="encrypt" \
    -e XOAUTH2_CLIENT_ID_FILE="/run/secrets/xoauth2-client-id" \
    -e XOAUTH2_SECRET_FILE="/run/secrets/xoauth2-secret" \
    -e ALLOW_EMPTY_SENDER_DOMAINS="true" \
    -e XOAUTH2_INITIAL_ACCESS_TOKEN_FILE="/run/secrets/xoauth2-access-token" \
    -e XOAUTH2_INITIAL_REFRESH_TOKEN_FILE="/run/secrets/xoauth2-refresh-token" \
    boky/postfix

Currently, this is only supported for RELAYHOST_PASSWORD, XOAUTH2_CLIENT_ID, XOAUTH2_SECRET, XOAUTH2_INITIAL_ACCESS_TOKEN and XOAUTH2_INITIAL_REFRESH_TOKEN.

Helm chart

This image comes with its own helm chart. The chart versions are aligned with the releases of the image. Charts are hosted through this repository.

To install the image, simply do the following:

helm repo add bokysan https://bokysan.github.io/docker-postfix/
helm upgrade --install --set persistence.enabled=false --set config.general.ALLOWED_SENDER_DOMAINS=example.com mail bokysan/mail

Chart configuration is as follows:

Property Default value Description
replicaCount 1 How many replicas to start
image.repository boky/postfix This docker image repository
image.tag empty Docker image tag, by default uses Chart's AppVersion
image.pullPolicy IfNotPresent Pull policy for the image
imagePullSecrets [] Pull secrets, if neccessary
nameOverride "" Override the helm chart name
fullnameOverride "" Override the helm full deployment name
serviceAccount.create true Specifies whether a service account should be created
serviceAccount.annotations {} Annotations to add to the service account
serviceAccount.name "" The name of the service account to use. If not set and create is true, a name is generated using the fullname template
service.type ClusterIP How is the server exposed
service.port 587 SMTP submission port
service.labels {} Additional service labels
service.annotations {} Additional service annotations
service.spec {} Additional service specifications
service.nodePort empty Use a specific nodePort
service.nodeIP empty Use a specific nodeIP
resources {} Pod resources
autoscaling.enabled false Set to true to enable Horisontal Pod Autoscaler
autoscaling.minReplicas 1 Minimum number of replicas
autoscaling.maxReplicas 100 Maximum number of replicas
autoscaling.targetCPUUtilizationPercentage 80 When to scale up
autoscaling.targetMemoryUtilizationPercentage "" When to scale up
autoscaling.labels {} Additional HPA labels
autoscaling.annotations {} Additional HPA annotations
nodeSelector {} Standard Kubernetes stuff
tolerations [] Standard Kubernetes stuff
affinity {} Standard Kubernetes stuff
certs.create {} Auto generate TLS certificates for Postfix
extraVolumes [] Append any extra volumes to the pod
extraVolumeMounts [] Append any extra volume mounts to the postfix container
extraInitContainers [] Execute any extra init containers on startup
extraEnv [] Add any extra environment variables to the container
extraContainers [] Add extra containers
deployment.labels {} Additional labels for the statefulset
deployment.annotations {} Additional annotations for the statefulset
pod.securityContext {} Pods's security context
pod.labels {} Additional labels for the pod
pod.annotations {} Additional annotations for the pod
container.postfix.securityContext {} Containers's security context
config.general {} Key-value list of general configuration options, e.g. TZ: "Europe/London"
config.postfix {} Key-value list of general postfix options, e.g. myhostname: "demo"
config.opendkim {} Key-value list of general OpenDKIM options, e.g. RequireSafeKeys: "yes"
secret {} Key-value list of environment variables to be shared with Postfix / OpenDKIM as secrets
existingSecret "" A reference to an existing opaque secret. Secret is mounted and exposed as environment variables in the pod
mountSecret.enabled false Create a folder with contents of the secret in the pod's container
mountSecret.path /var/lib/secret Where to mount secret data
mountSecret.data {} Key-value list of files to be mount into the container
persistence.enabled true Persist Postfix's queue on disk
persistence.accessModes [ 'ReadWriteOnce' ] Access mode
persistence.existingClaim "" Provide an existing PersistentVolumeClaim, the value is evaluated as a template.
persistence.size 1Gi Storage size
persistence.storageClass "" Storage class
recreateOnRedeploy true Restart Pods on every helm deployment, to prevent issues with stale configuration(s).

Metrics

You may enable metrics on the cart by simply setting metrics.enabled=true. Of course, this comes with some caveats, namely:

Please see helm chart's values.yaml for further configuration options and how to enable ServiceMonitor, if you need it for Prometheus.

Extending the image

Using custom init scripts

If you need to add custom configuration to postfix or have it do something outside of the scope of this configuration, simply add your scripts to /docker-init.db/: All files with the .sh extension will be executed automatically at the end of the startup script.

E.g.: create a custom Dockerfile like this:

FROM boky/postfix
LABEL maintainer="Jack Sparrow <jack.sparrow@theblackpearl.example.com>"
ADD Dockerfiles/additional-config.sh /docker-init.db/

Build it with docker, and your script will be automatically executed before Postfix starts.

Or -- alternately -- bind this folder in your docker config and put your scripts there. Useful if you need to add a config to your postfix server or override configs created by the script.

For example, your script could contain something like this:

#!/bin/sh
postconf -e "address_verify_negative_cache=yes"

Security

Postfix will run the master proces as root, because that's how it's designed. Subprocesses will run under the postfix and opendkim accounts.

UIDs/GIDs numbers

While I cannot guarantee IDs (they are auto-generated by package manages), they tend to be fairly consistent across specific distribution. Please be aware of this if you are switching images from Alpine to Debian to Ubuntu or back.

At the last check, images had the following UIDs/GIDs:

Service Debian (UID/GID) Ubuntu (UID/GID) Alpine (UID/GID)
postfix 100:102 101:102 100:101
opendkim 101:104 102:104 102:103

Please check the notification information on startup.

Quick how-tos

Relaying messages through your Gmail account

Please note that Gmail does not support using your password with non-OAuth2 clients. You will need to either enable Less secure apps in your account and assign an "app password", or configure postfix support for XOAuth2 authentication. You'll also need to use (only) your email as the sender address.

If you follow the less than secure route, your configuration would be as follows:

RELAYHOST=smtp.gmail.com:587
RELAYHOST_USERNAME=you@gmail.com
RELAYHOST_PASSWORD=your-gmail-app-password
ALLOWED_SENDER_DOMAINS=gmail.com

There's no need to configure DKIM or SPF, as Gmail will add these headers automatically.

Relaying messages through Google Apps account

Google Apps allows third-party services to use Google's SMTP servers without much hassle. If you have a static IP, you can configure Gmail to accept your messages. You can then send email from any address within your domain.

You need to enable the SMTP relay service:

Add setting SMTP relay service

Your configuration would be as follows:

RELAYHOST=smtp-relay.gmail.com:587
ALLOWED_SENDER_DOMAINS=<your-domain>

There's no need to configure DKIM or SPF, as Gmail will add these headers automatically.

Relaying messages through Amazon's SES

If your application runs in Amazon Elastic Compute Cloud (Amazon EC2), you can use Amazon SES to send up to 62,000 emails every month at no additional charge. You'll need an AWS account and SMTP credentials. The SMTP settings are available on the SES page.

For example, for eu-central-1:

You need Amazon SES SMTP credentials to access the SES SMTP interface. NOTE: Your SMTP password is different from your AWS secret access key. Additionally, the credentials that you use to send email through the SES SMTP interface are unique to each AWS Region. If you use the SES SMTP interface to send email in more than one Region, you must generate a set of SMTP credentials for each Region that you plan to use.

Make sure you write the user credentials down, as you will only see them once.

By default, messages that you send through Amazon SES use a subdomain of amazonses.com as the MAIL FROM domain. See Amazon's documentation on how the domain can be configured.

Your configuration would be as follows (example data, these key will not work):

RELAYHOST=email-smtp.eu-central-1.amazonaws.com:587
RELAYHOST_USERNAME=AKIAGHEVSQTOOSQBCSWQ
RELAYHOST_PASSWORD=BK+kjsdfliWELIhEFnlkjf/jwlfkEFN/kDj89Ufj/AAc
ALLOWED_SENDER_DOMAINS=<your-domain>

You will need to configure DKIM and SPF for your domain as well.

Sending messages directly

If you're sending messages directly, you'll need to:

Your configuration would be as follows:

ALLOWED_SENDER_DOMAINS=<your-domain>

Careful

Getting all of this to work properly is not a small feat:

Similar projects

There are may other project offering similar functionality. The aim of this project, however, is:

The other projects are, in completely random order:

License check

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