MailCrab

Email test server for development, written in Rust.

MailCrab was created as an exercise in Rust, trying out Axum and functional components with Yew, but most of all because it is really enjoyable to write Rust code.

TLDR

docker run --rm -p 1080:1080 -p 1025:1025 marlonb/mailcrab:latest

Features

Accept-all SMTP server
Web interface to view and inspect all incoming email
View formatted mail, download attachments, view headers or the complete raw mail contents
Single binary
Runs on all amd64 and arm64 platforms using docker
Just a 7.77 MB docker image

Related projects

CrabAlert is a macOS status bar application that notifies you of incoming messages in MailCrab

Technical overview

Both the backend server and the frontend are written in Rust. The backend receives email over an unencrypted connection on a configurable port. All email is stored in memory while the application is running. An API exposes all received email:

GET /api/messages return all message metadata
GET /api/message/[id] returns a complete message, given its id
POST /api/delete/[id] deletes a message, given its id
POST /api/delete-all deletes all messages
GET /api/version returns version information about the executable
GET /ws send email metadata to each connected client when a new email is received

The frontend initially performs a call to /api/messages to receive all existing email metadata and then subscribes for new messages using the websocket connection. When opening a message, the /api/message/[id] endpoint is used to retrieve the complete message body and raw email.

The backend also accepts a few commands over the websocket, to mark a message as opened, to delete a single message or delete all messages.

Installation and usage

You can run MailCrab using docker. Start MailCrab using the following command:

docker run --rm -p 1080:1080 -p 1025:1025 marlonb/mailcrab:latest

Open a browser and navigate to http://localhost:1080 to view the web interface.

There are also (single) binary builds available, see https://github.com/tweedegolf/mailcrab/releases

Ports

The default SMTP port is 1025, the default HTTP port is 1080. You can configure the SMTP and HTTP port using environment variables (SMTP_PORT and HTTP_PORT), or by exposing them on different ports using docker:

docker run --rm -p 3000:1080 -p 2525:1025 marlonb/mailcrab:latest

Host

You can specify the host address MailCrab will listen on for HTTP request using the HTTP_HOST environment variable. In the docker image the default address is 0.0.0.0, when running MailCrab directly using cargo or a binary, the default is 127.0.0.1.

TLS

You can enable TLS and authentication by setting the environment variable ENABLE_TLS_AUTH=true. MailCrab will generate a key-pair and print the self-signed certificate. Any username/password combination is accepted. For example:

docker run --rm --env ENABLE_TLS_AUTH=true -p 1080:1080 -p 1025:1025 marlonb/mailcrab:latest

It is also possible to provide your own certificate by mounting a key and a certificate to /app/key.pem and /app/cert.pem:

docker run --rm --env ENABLE_TLS_AUTH=true -v key.pem:/app/key.pem:ro -v cert.pem:/app/cert.pem:ro -p 1080:1080 -p 1025:1025 marlonb/mailcrab:latest

Path prefix

You can configure a prefix path for the web interface by setting and environment variable named MAILCRAB_PREFIX, for example:

docker run --rm --env MAILCRAB_PREFIX=emails -p 1080:1080 -p 1025:1025 marlonb/mailcrab:latest

The web interface will also be served at http://localhost:1080/emails/

Reverse proxy

See the reverse proxy guide.

Retention period

By default messages will be stored in memory until MailCrab is restarted. This might cause an OOM when MailCrab lives long enough and receives enough messages.

By setting MAILCRAB_RETENTION_PERIOD to a number of seconds, messages older than the provided duration will be cleared.

Performance

MailCrab is fast, although there is a bottleneck in the throughput of the websocket connection (between the server and the browser). If there are many messages sent at once (more than 100 per second) a client can lag behind and messages can get lost. When dealing with many messages at once, increasing the internal queue size can help to prevent losing messages. Use the QUEUE_CAPACITY environment variable to set the queue size. De default is 32, which means that MailCrab can handle 32 messages if the are all sent at the same time.

docker compose

Usage in a docker-compose.yml file:

version: '3.8'
services:
  mailcrab:
    image: marlonb/mailcrab:latest
    #        environment:
    #            ENABLE_TLS_AUTH: true # optionally enable TLS for the SMTP server
    #            MAILCRAB_PREFIX: emails # optionally prefix the webinterface with a path
    #        volumes:
    #           key.pem:/app/key.pem:ro # optionally provide your own keypair for TLS, else a pair will be generated
    #           cert.pem:/app/cert.pem:ro
    ports:
      - '1080:1080'
      - '1025:1025'
    networks: [default]

Kubernetes deployment

To deploy MailCrab to a Kubernetes cluster, you can use Helm Chart by cloning this repository and running:

helm install mailcrab ./charts/mailcrab -f values.yaml

For more information on configuring the Helm Chart, see the chart README.

Sample messages

The samples directory contains a couple of test messages. These can be sent using by running:

cd backend/
cargo test send_sample_messages -- --ignored

Alternatively you can send messages using curl:

curl smtp://127.0.0.1:1025 --mail-from myself@example.com --mail-rcpt receiver@example.com --upload-file samples/normal.email
# with tls
curl -k --ssl-reqd smtps://127.0.0.1:1025 --mail-from myself@example.com --mail-rcpt receiver@example.com --upload-file samples/normal.email --user 'user:pass'

Development

Install Rust and Trunk

# Add wasm as target if it it not present after following the install instructions for Trunk
rustup target add wasm32-unknown-unknown

# clone the code
git clone git@github.com:tweedegolf/mailcrab.git

# start the backend
cd backend
cargo run

# serve the frontend (in a new terminal window)
cd ../frontend
trunk serve

# optionally send test messages in an interval
cd ../backend
cargo test