Inbucket is described as 'Need to test your webapps outbound emails with Mailinator but stuck behind a firewall? Want to keep the new application you are developing secret until it’s time to release it? Need to validate that emails go out as part of your integration test suite?' and is an app in the Office & Productivity category. There are five alternatives to Inbucket for a variety of platforms, including Linux, Mac, Windows, Online / Web-based and BSD. The best alternative is Postfix, which is both free and Open Source. Other great apps like Inbucket are MailHog (Free, Open Source), OpenSMTPD (Free, Open Source), Go-Guerrilla (Free, Open Source) and Lettre (Free, Open Source).
Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail. It attempts to be fast, easy to administer, and secure. The software is also known by its former names VMailer and IBM Secure Mailer.
OpenSMTPD is a FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard extensions. It allows ordinary machines to exchange e-mails with other systems speaking the SMTP protocol.