Mailpit

Mailpit is a free, open-source email testing utility built around a local SMTP server, a web interface, and an integration-testing API. It is best suited to developers who need to inspect and exercise email flows in development or CI/CD without using real recipients. Its testing toolkit is broad; its default retention and unsecured SMTP setup need attention.

Overview

Mailpit captures application email locally so developers can inspect messages and test delivery behavior before messages reach real inboxes. It runs as a single binary or a multi-architecture Docker image without additional dependencies, making it practical to add to a workstation or CI/CD pipeline. It is a development and testing tool, not a production email delivery service.

By default, SMTP listens on port 1025 without encryption or authentication. That is convenient for a local setup, but should not be treated as a safe network-facing configuration. The web UI and API can use HTTPS and basic authentication, while SMTP can be configured for authentication and TLS.

Key features

Inspect messages and catch email defects

The web interface displays formatted HTML, highlighted HTML source, text, headers, raw source, and MIME attachments with image thumbnails. This makes it useful for tracing both rendering issues and problems in the message itself. HTML Check compares messages against more than 175 HTML and CSS checks based on compatibility data from caniemail.com. Link checks, HTML screenshots, and optional SpamAssassin-based spam checks add further ways to assess a message before release.

Test delivery behavior and automate workflows

Mailpit can tag messages, relay them through another SMTP server, forward them automatically, or trigger configurable SMTP errors with its Chaos feature. That error simulation is particularly useful for checking how an application handles failed delivery rather than only verifying the happy path.

The REST API supports accessing, searching, deleting, sending, tagging, and releasing messages, which makes Mailpit suitable for integration tests. It can also send message-summary webhooks to external services; it has been tested with n8n and offers a Cypress package for integration tests.

Consider safety and capacity

The UI applies a strict Content-Security-Policy and a no-referrer policy, though tracking images remain allowed. Link checking blocks requests to internal networks by default to help prevent server-side request forgery. Mailpit reports SMTP ingestion of 100–200 emails per second depending on system and message factors, but its defaults are oriented toward development: it retains the most recent 500 messages, allows up to 100 recipients per message, and optional POP3 exposes only the latest 100 messages.

Pricing

Mailpit is free and open source. 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 is no free trial because the product is free. This is a strong fit for developers who want self-hosted email testing without a subscription; the trade-off is that the default message retention is finite, and POP3 access is limited to the latest 100 messages.

Platforms

Mailpit supports API, Linux, macOS, self-hosted, web, and Windows use. Static binaries are available for Windows, Linux, and Mac, while Docker images cover 386, amd64, and arm64. This range gives teams a choice between installing a binary and running a container across common development environments.

Who it's for

Mailpit is a good choice for developers who need a local SMTP destination, message inspection, and automated checks in development or CI/CD. Teams that need to test failure handling benefit from its configurable SMTP errors, while those debugging templates can use its HTML and link checks. It is less suitable when the need is production email delivery or a default SMTP configuration that is already secured for network exposure.

Pros and cons

• Pros: A single free package combines local SMTP capture, detailed message inspection, HTML and link checks, screenshots, and optional spam scoring.
• Pros: The REST API, webhooks, and Cypress package support automated integration workflows, while configurable SMTP errors help test resilience.
• Pros: Binary and Docker deployment options, plus Windows, Linux, and Mac support, make it adaptable to different development setups.
• Cons: Default SMTP has no encryption or authentication, so users need to configure security before exposing it beyond a trusted local environment.
• Cons: The default 500-message retention and 100-message POP3 window constrain longer-running or POP3-based workflows.

Alternatives

Compare email testing software if you want to consider a wider set of tools. MailHog is another free, open-source email testing tool for API, Linux, macOS, self-hosted, and web use. Choose MailSlurp if its free plan's monthly allowances—500 received emails, 1,000 sandbox sends, one device render, and one inbox placement test—match your needs. Mailtrap Email Sandbox offers a free plan capped at 50 test emails per month, one user, and one sandbox, so it may suit a small, quota-based sandbox workflow. Emailens has a free plan with 30 simulated previews per day and 21 email clients, making it an option when simulated previews are the priority. GlockApps offers a free plan with two spam test credits and DMARC and uptime-monitoring allowances, which may suit readers focused on those checks. SMTP Labs includes SMTP, IMAP, POP3, and API on its free plan, along with one domain, 50 accounts, and seven-day message retention. Unspam.email offers a no-account free plan with three spam tests, two inbox placements, one device preview, and 24-hour data retention. Twilio Segment is a freemium web, iOS, and Android platform with a free plan that includes visitor, Reverse ETL, source, and integration allowances.

Verdict

Choose Mailpit if you want a free, self-hosted way to capture, inspect, and automate email tests across development and CI/CD. Its most compelling advantage is the combination of deep message inspection and test controls in a lightweight deployment. Look elsewhere for production delivery, or configure security and retention deliberately if your setup reaches beyond local development.