smtp4dev is described as 'Dummy SMTP server that sits in the system tray and does not deliver the received messages. The received messages can be quickly viewed, saved and the source/structure inspected. Useful for testing/debugging software that generates email' and is an app in the Development category. There are more than 10 alternatives to smtp4dev for a variety of platforms, including Online / Web-based, Windows, SaaS, Linux and Mac. The best alternative is Ethereal Email, which is free. Other great apps like smtp4dev are Mailtrap (Freemium), FakeSMTP (Free, Open Source), Origami SMTP (Free, Open Source) and Papercut (Free, Open Source).
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.
QA tool for automating end-to-end email tests. Get unlimited private email addresses and inboxes for testing new user signups, transactional emails and drip campaigns, deliverability and spam scores, etc. Integrates with your CI/CD workflow. Includes spam testing.
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.