Mailpit

Mailpit is a free, open-source SMTP and email testing tool for developers. It is best suited to local development and CI/CD workflows that need to inspect messages and automate delivery scenarios. Its broad testing toolkit is useful, but the default message cap and unsecured SMTP listener need attention.

Overview

Mailpit combines a local SMTP server, a web interface for inspecting messages, and an API for integration testing. It can run as a single binary or a multi-architecture Docker image without additional dependencies, making it straightforward to include in a development environment or pipeline. It is a testing tool, not a production mail-delivery service: its purpose is to let developers exercise email functionality without sending messages to real addresses.

Key features

Inspect and check email

The web interface presents formatted HTML, highlighted HTML source, text, headers, raw source, and MIME attachments, including image thumbnails. That range helps developers distinguish a presentation issue from a problem in the underlying message. HTML Check applies more than 175 HTML and CSS checks based on compatibility data from caniemail.com. Link checks and HTML screenshots add useful checks before release, while optional SpamAssassin-based spam scoring offers another signal for message testing.

Test delivery behavior and automate

Messages can be tagged, relayed through another SMTP server, automatically forwarded, or used to trigger configurable SMTP errors with the Chaos feature. This makes Mailpit useful for checking how an application handles delivery failures as well as for inspecting successful output. Its REST API supports accessing, searching, deleting, sending, tagging, and releasing messages, which lends itself to repeatable integration tests. Message-summary webhooks can notify external services; Mailpit has been tested with n8n and offers a Cypress package for integration tests.

Security and operational limits

The web UI and API can use HTTPS and basic authentication, while SMTP supports optional authentication and TLS. The UI applies a strict Content-Security-Policy and a no-referrer policy, though it still allows tracking images. Link checking blocks requests to internal networks by default, helping guard against server-side request forgery. These controls are configurable rather than the default SMTP posture: SMTP listens on port 1025 without encryption or authentication unless changed. Configure it carefully wherever other users or services can reach the listener.

Mailpit reports SMTP ingestion of 100–200 emails per second depending on system and message factors, but its default retention keeps only the most recent 500 messages. The optional POP3 service exposes only the latest 100 for performance reasons, and the SMTP server allows up to 100 recipients per message by default. These defaults suit bounded test workloads better than long-term message archiving.

Pricing

Mailpit is free and open source, with no free trial. Its Mailpit plan costs nothing and includes a single static binary or multi-architecture Docker images, with default retention of the most recent 500 emails. There are no paid tiers or seat charges to weigh; the practical trade-off is that the free plan's default retention is limited, so teams with larger test volumes should account for pruning in their workflow.

Platforms

Mailpit supports API, Linux, macOS, self-hosted, web, and Windows use. Static binaries are provided for Windows, Linux, and Mac, and Docker images cover 386, amd64, and arm64. Its single-binary and Docker deployment options make it suitable for both a developer workstation and a self-hosted test environment.

Who it's for

Mailpit is a strong fit for developers who need to capture application email locally, inspect its structure and rendering, or test delivery behavior without contacting real recipients. Its API, Cypress package, and webhook support also suit teams adding email scenarios to integration testing or CI/CD. It is less suitable for anyone seeking a production sending platform, a long-retention archive, or SMTP that is secure by default without configuration.

Pros and cons

Pros: Detailed message inspection spans rendered content, source, headers, and attachments, helping pinpoint defects across the whole email.

Pros: HTML, link, spam, and screenshot checks provide several ways to catch email problems before delivery.

Pros: REST API and configurable SMTP errors support automated tests of both normal and failure paths.

Pros: Free, open-source deployment as a standalone binary or Docker image avoids an additional service dependency.

Cons: SMTP begins without encryption or authentication, so users must configure safeguards when the listener is exposed beyond a trusted test environment.

Cons: The default 500-message retention and 100-message POP3 window constrain how much history can be kept or accessed.

Cons: Tracking images remain allowed in the UI despite its other rendering protections.

Alternatives

Email Testing Software is the broader category to compare if Mailpit's local, self-hosted testing model is not the right fit.

Choose Twilio Segment instead if web, iOS, or Android platform support and a free plan with visitor and Reverse ETL quotas are a better match. For real-device email rendering, EmailQA offers a free tier with three projects, two team members, 25 renders per month, and seven-day render history, plus a Pro plan at $20 a month. Mailinator is another freemium alternative with private inbox and domain allowances on its Verified Pro free plan.

If spam and domain reputation monitoring is the priority, GlockApps has a free plan with two spam test credits, DMARC monitoring, uptime monitors, and one user. RenderOps may suit developers looking for a free set of client-side email utilities. MailSlurp instead offers a freemium API service with monthly email, sandbox-send, render, and inbox-creation quotas. For another free, open-source email testing tool, consider MailHog. Mailtrap Email Sandbox is a freemium alternative with a free sandbox capped at 50 test emails per month, one user, and one sandbox.

Verdict

Choose Mailpit if you want a free, self-hostable way to capture, inspect, and automate email tests without sending messages to real recipients. Its inspection and testing breadth make it particularly useful in development and CI/CD. Look elsewhere if you need a production mail service or a larger default message history, and configure SMTP security before making the listener reachable to untrusted systems.