SubEthaSMTP is an easy to understand Java library that provides a receptive SMTP server component. By plugging this component into your Java application, you can easily receive SMTP mail using a simple abstract Java interface. Also included is a small sub-project called Wiser, an easy to use incoming mail testing framework.
j-XChange is a pure Java implementation of the entire Collaboration Data Objects (CDO 1.21) library for accessing Microsoft Exchange Server in a platform independent manner. This library will allow interoperability with MS Exchange from all non-Windows platforms that support Java. This library is powered by j-Interop.
GreenMail is a suite of email servers for testing purposes and sandbox development. It supports SMTP, POP3, and IMAP (including SSL) and can be embedded into any Java application or unit test. The GreenMail Service (SAR) extension runs as a JBoss embedded developer mail server sandbox, manageable via JMX.
Java SMTP Mail Component provides an easy API to send email from an application. The API has option to send emails directly to the recipients mail server without a relaying mail server. Plain authentication is supported for mail server authentication. Mail attachments are supported using MIME.