Mailtrap is described as 'Email Delivery Platform for businesses and individuals to Test, Send, and Control their email infrastructure performance' and is a Mail Server in the development category. There are more than 25 alternatives to Mailtrap, not only websites but also apps for a variety of platforms, including Mac, Windows, Linux and SaaS apps. The best Mailtrap alternative is MailHog, which is both free and Open Source. Other great sites and apps similar to Mailtrap are Mailpit, Mailcatch.app, FakeSMTP and MailMug.net.
MailPit is the continuation of this product.