deweysasser/docker-postfix repository overview

⁠docker-postfix

Simple Postfix SMTP TLS relay docker⁠ alpine based image with no local authentication enabled (to be run in a secure LAN).

It also includes rsyslog to enable logging to stdout.

If you want to follow the development of this project check out my blog⁠.

⁠Available image tags

Currently we only handle a rolling release of new versions so only latest tag is available, but there is work in progress⁠ to start releasing versioned images to be able to pin to specific versions in production deployments.

NOTES:

• The alpine tag has been switched to use the master branch, but it's irrelevant as it is the same as latest.
• Old CentOS 7 based image is avaiable on the centos_base_image branch, but it is not being developed any more.

⁠Build instructions

Clone this repo and then:

cd docker-Postfix
sudo docker build -t juanluisbaptiste/postfix .

Or you can use the provided docker-compose⁠ files:

sudo docker-compose build

For more information on using multiple compose files see here⁠. You can also find a prebuilt docker image from Docker Hub⁠, which can be pulled with this command:

sudo docker pull juanluisbaptiste/postfix:latest

⁠How to run it

The following env variables need to be passed to the container:

• SMTP_SERVER Server address of the SMTP server to use.
• SMTP_PORT (Optional, Default value: 587) Port address of the SMTP server to use.
• SMTP_USERNAME (Optional) Username to authenticate with.
• SMTP_PASSWORD (Mandatory if SMTP_USERNAME is set) Password of the SMTP user. If SMTP_PASSWORD_FILE is set, not needed.
• SERVER_HOSTNAME Server hostname for the Postfix container. Emails will appear to come from the hostname's domain.

The following env variable(s) are optional.

• SMTP_HEADER_TAG This will add a header for tracking messages upstream. Helpful for spam filters. Will appear as "RelayTag: ${SMTP_HEADER_TAG}" in the email headers.

• SMTP_NETWORKS Setting this will allow you to add additional, comma seperated, subnets to use the relay. Used like -e SMTP_NETWORKS='xxx.xxx.xxx.xxx/xx,xxx.xxx.xxx.xxx/xx'

• SMTP_PASSWORD_FILE Setting this to a mounted file containing the password, to avoid passwords in env variables. Used like -e SMTP_PASSWORD_FILE=/secrets/smtp_password -v $(pwd)/secrets/:/secrets/

• ALWAYS_ADD_MISSING_HEADERS This is related to the always_add_missing_headers⁠ Postfix option (default: no). If set to yes, Postfix will always add missing headers among From:, To:, Date: or Message-ID:.

• OVERWRITE_FROM This will rewrite the from address overwriting it with the specified address for all email being relayed. Example settings: OVERWRITE_FROM=[email protected]⁠ OVERWRITE_FROM="Your Name" [email protected]⁠

To use this container from anywhere, the 25 port or the one specified by SMTP_PORT needs to be exposed to the docker host server:

docker run -d --name postfix -p "25:25"  \
       -e SMTP_SERVER=smtp.bar.com \
       -e [email protected] \
       -e SMTP_PASSWORD=XXXXXXXX \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \
       juanluisbaptiste/postfix

If you are going to use this container from other docker containers then it's better to just publish the port:

docker run -d --name postfix -P \
       -e SMTP_SERVER=smtp.bar.com \
       -e [email protected] \
       -e SMTP_PASSWORD=XXXXXXXX \
       -e SERVER_HOSTNAME=helpdesk.mycompany.com \           
       juanluisbaptiste/postfix

Or if you can start the service using the provided docker-compose⁠ file for production use:

sudo docker-compose up -d

To see the email logs in real time:

docker logs -f postfix

⁠A note about using gmail as a relay

Gmail by default does not allow email clients that don't use OAUTH 2⁠ for authentication (like Thunderbird or Outlook). First you need to enable access to "Less secure apps" on your google settings⁠.

Also take into account that email From: header will contain the email address of the account being used to authenticate against the Gmail SMTP server(SMTP_USERNAME), the one on the email will be ignored by Gmail unless you add it as an alias⁠.

⁠Debugging

If you need troubleshooting the container you can set the environment variable DEBUG=yes for a more verbose output.