Docker Intro using MailCatcher

Safely Test email sending in a dev environment

Bill Tihen

Last updated on Sun, 05 May 2024 Technology, Docker

Intro

It is often helpful to be able to test email sending from an application during development or testing (or even to make testing emails on a staging server possible).

To do this follow these instructions for a safe convenient way to test and inspect emails sent from an application.

SETUP

First we need to get the repo (or at least the docker file)

# get the mailcatcher repo
git clone git@github.com:sj26/mailcatcher.git

# go into mailcather repo
cd mailcatcher

# configure to use the newest `released` gem version of mailcatcher
sed -i.bu1 's/FROM ruby:2.5/FROM ruby:2.6/' Dockerfile
sed -i.bu2 's/ARG VERSION=0.6.5/ARG VERSION=0.7.1/' Dockerfile

The Dockerfile should now look like (which is actually all that is actually needed):

FROM ruby:2.6
MAINTAINER Samuel Cochran <sj26@sj26.com>

ARG VERSION=0.7.1

RUN gem install mailcatcher -v $VERSION

EXPOSE 1025 1080

ENTRYPOINT ["mailcatcher", "--foreground"]
CMD ["--ip", "0.0.0.0"]

BUILD IMAGE

Now you can download the docker image and install the gems into it with:

# -t adds repository:tag info -- the '.' at the end is important:
docker build -t btihen/ruby/mailcatcher:ruby_2.6 .
# ...
# should end with something like
# Successfully built 21e0de2bdd68

# now tag it as the **lasted** image with:
docker build -t btihen/ruby/mailcatcher:latest .

now you can see your list of docker images (you should see the starting image/container we just created):

docker images
REPOSITORY                 TAG                 IMAGE ID            CREATED             SIZE
btihen/ruby/mailcatcher    latest              21e0de2bdd68        8 minutes ago       870MB
btihen/ruby/mailcatcher    ruby_2.6            21e0de2bdd68        8 minutes ago       870MB
ruby                       2.6                 a98425292e84        2 weeks ago         843MB

BUILD CONTAINER

Now start the docker image using the build image id (-d allows it to run in the backgroud, -p 1025:1025 -p 1080:1080 opens a connection on ports 1025 & 1080 from localhost to the docker image):

# build a container so we can test our image
docker run -d -p 1025:1025 -p 1080:1080 --name mailcatcher btihen/ruby/mailcatcher:latest

# or if you like ids better
docker run -d -p 1025:1025 -p 1080:1080 --name mailcatcher 21e0de2bdd68

# if you forgot the image-id you can list the images with:
docker images

TESTING (http & smtp)

now you should be able to go to http://localhost:1080 and see the mailcatcher webpage.

now lets test the smtp side from the cli using these instructions: https://www.shellhacks.com/send-email-smtp-server-command-line/

# connect to the mail server
$ telnet localhost 1025
# or
$ telnet 127.0.0.1 1025
220 smtp.domain.ext ESMTP Sendmail ?version-number?; ?date+time+gmtoffset?

# declare yourself (IP or DNS)
> HELO local.domain.name
250 smtp.domain.ext Hello local.domain.name [xxx.xxx.xxx.xxx], pleased to meet you

# declare who the email is from:
> MAIL FROM: test@local.domain.name
250 2.1.0 sender@adress.ext... Sender ok

# declare who should get the email:
> RCPT TO: recipient@adress.ext
250 2.1.5 recipient@adress.ext... Recipient ok

# setup the DATA transmission:
 > DATA
354 Enter mail, end with "." on a line by itself

# type a subject two returns and a message ending with '.' (on its own line):
SUBJECT: Test message

Hello,
this is a TEST message,
please don't reply.
Thank you.
.

# end the connection
> QUIT

Now check the mail has arrived in mailcatcher at localhost:1080

Assuming you see the email sent - you can be sure your image & container is setup properly.

STOPPING (exited) CONTAINER

When we are done with mailcatcher we can stop the docker process:

docker ps -a
docker kill mailcatcher

STARTING BUILT (but exited) CONTAINERS

To restart mailcatcher at a later date simply type:

docker start mailcatcher

# login to the Azure Container Repository
docker login btihen -u username -p xxxxxxxxxxx

# upload the new image
docker push btihen/ruby/mailcatcher

RETRIEVING SHARED IMAGE

az acr login --name username
az acr repository list --name username --output table

# getting the image
docker pull btihen/ruby/image_name

containerize the image

# these are the default local ports - adjust to your needs
docker run -d -p 1025:1025 -p 1080:1080 --name mailcatcher btihen/ruby/mailcatcher:latest

start the container

docker start mailcatcher

LISTING Repo IMAGES

One-time install

# if needed install the azure cli
brew update && brew install azure-cli

# the following may also be needed:
brew update && brew install python3 && brew upgrade python3
brew link --overwrite python3

Retrieve the image list

# login with the azure-cli
az acr login --name username

# list the images
az acr repository list --name username --output table

REMOVING CONTAINERS

when we no longer need mailcatcher we can remove it with (-a lists running and stopped containers):

docker ps -a
docker rm mailcatcher

REMOVING IMAGES To fully clean up and remove (images – after the containers are removed):

$ docker images
REPOSITORY               TAG                 IMAGE ID            CREATED             SIZE
btihen/ruby/mailcatcher  ruby_2.5            21e0de2bdd68        25 minutes ago      870MB
ruby                     2.5                 a98425292e84        2 weeks ago         843MB

$ docker image rm 21e0de2bdd68

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ruby                2.5                 a98425292e84        2 weeks ago         843MB

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