FakeSMTP is a lightweight and portable SMTP server, designed to help developers and testers test their applications and websites that use an SMTP server for sending emails. It is written in Java and can be used on any platform with a Java Runtime Environment (JRE). It is available as both a stand-alone application, as well as a web application. FakeSMTP allows users to send, receive and monitor emails, as well as manage their email accounts. It also includes features such as message spoofing and customizable headers. FakeSMTP also allows users to simulate sending/receiving multiple emails and to capture email data such as IP addresses, headers, attachments, etc.
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Very good GUI on Windows with many configuration options and visible headers, etc.