The best open source alternative to Mailtrap is MailHog, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Mailtrap and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Mailtrap are FakeSMTP (Free, Open Source), Papercut (Free, Open Source), Origami SMTP (Free, Open Source) and Sendria (Free, Open Source).
MailHog is an email testing tool inspired by MailCatcher, but easier to install. - Configure your application to use MailHog for SMTP delivery - View emails in the web UI, or through JSON API - Release messages to real SMTP servers for delivery
FakeSMTP is a Free Fake SMTP Server with GUI for testing emails in applications easily. It is written in Java. Configure your application to use "localhost" as your SMTP server, and all emails will be intercepted and displayed in this software.
Origami SMTP was created with the idea to have a fake email server for testing purposes that supported TLS. This is because the code the developer was working on at the time had several hard coded values enforcing TLS so most other solutions would not work.
Sendria (formerly MailTrap) is a SMTP server designed to run in your dev/test environment, that is designed to catch any email you or your application is sending, and display it in a web interface instead of sending to real world.
LunaticSMTP is a dummy SMTP server with desktop GUI for testing email sending applications. To use it, start LunaticSMTP and configure your application to use 'localhost' as it's SMTP server. Now it will catch messages sent to it and allow you to view them.
MailCatcher runs a super simple SMTP server which catches any message sent to it to display in a web interface. Run mailcatcher, set your favourite app to deliver to smtp://127.0.0.1:1025 instead of your default SMTP server, then check out...