mailcrab
A lightweight email test server for development with a web interface, written in Rust.
What is mailcrab?
MailCrab is an email test server designed for development environments that captures and inspects emails without sending them to real addresses. It provides a web interface to view, inspect, and manage all incoming emails, including attachments and headers, making it an essential tool for testing email functionality in applications.
Developers and QA engineers working on applications that send emails, who need a local or development environment tool to test email functionality without spamming real addresses.
Developers choose MailCrab for its lightweight, single-binary design and Docker support, offering a minimal 7.77 MB image with real-time WebSocket updates and configurable retention, making it easy to integrate into development workflows without unnecessary complexity.
Overview
Email test server for development, written in Rust
Use Cases
Best For
- Testing email functionality in web applications during development without sending real emails.
- Capturing and inspecting SMTP emails, including headers and attachments, in a local development environment.
- Integrating a lightweight email test server into Docker-based development stacks using a minimal Docker image.
- Real-time monitoring of incoming emails via WebSocket connections for immediate feedback during testing.
- Deploying an email test server in Kubernetes clusters using the provided Helm chart for scalable testing environments.
- Configuring TLS and authentication for secure email testing in development with optional self-signed or custom certificates.
Not Ideal For
- High-volume, continuous integration testing where thousands of emails are sent over extended periods, due to in-memory storage risks and WebSocket bottlenecks.
- Teams needing realistic email delivery simulation, such as testing SMTP failures, bounces, or authentication rejections, since MailCrab accepts all emails without validation.
- Environments requiring persistent email storage across server restarts, as messages are only stored in memory unless retention is configured.
Pros & Cons
Pros
As a single binary with a 7.77 MB Docker image, MailCrab is incredibly easy to deploy and run in any development environment, minimizing setup overhead.
WebSocket connections provide instant notifications of new emails, allowing developers to see test results immediately without refreshing the web interface.
The web interface displays formatted emails, headers, attachments, and raw content, making it easy to debug email content and structure in detail.
With support for TLS, configurable retention periods, path prefixes, and Docker/Kubernetes deployment via Helm, MailCrab adapts to various development setups.
Cons
Storing all emails in memory can cause out-of-memory errors if retention isn't set, and the WebSocket bottleneck limits throughput to about 100 messages per second, risking message loss.
As an accept-all server, it doesn't emulate real-world email server behaviors like rejections or delays, which might be needed for thorough testing of edge cases.
Enabling TLS requires managing self-signed certificates or providing custom ones, adding complexity for secure testing scenarios compared to tools with built-in options.
Open Source Alternative To
mailcrab is an open-source alternative to the following products:
MailHog is an email testing tool for developers that captures outgoing SMTP emails from applications and provides a web interface to view them.
MailCatcher is a development tool that runs a simple SMTP server to catch outgoing mail from applications and displays it in a web interface for testing.
Frequently Asked Questions
Related Projects
check-if-email-existsCheck if an email address exists without sending any email, written in Rust. Comes with a ⚙️ HTTP backend.
lettrea mailer library for Rust
meliterminal mail client, mirror of https://git.meli-email.org/meli/meli.git https://crates.io/crates/meli
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