SMTP and POP3 email server. [Freeware, source available]

A full-featured SMTP/POP3/IMAP4 server with pluggable mail filters and agents for application processing. [Shareware]

Java mail server. Uses a JDBC database (free database InstantDB is provided) to store messages. [Open Source, GPL]

Fully-featured mail server which handles both transport and storage, and allows for easy extensibility using MailServlets (for example: send mails as fax or voice, filter mailets, translator, mailling list, newspaper via mail). [Open Source, BSD-like]

Very simple smtp pop3 java email server implementation with source. [Open Source,GPL]

Newsletter software capable of publishing e-mail newsletter and maintaining the subscribers. This newsletter software written in Java, provides stats and analysis for the published newsletters. [Commercial]

Server which allows access to POP3 mailboxes for users who don't have direct access to them. It can be used for accessing POP3 mailboxes from multiple computers even if only one of them has a dial-up connection to Internet. [Freeware]

A very simple example of a POP3 server in java. [Open source]

SMTP mail relay. [Freeware]