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.
Spam Trainer makes it easier for GNOME users to train spam filtering software (e.g. SpamBayes or SpamAssassin) that "learns" how to tell the difference between real email (ham) and unsolicited messages (spam). Not all email clients have spam filtering (and training) built in, and training external filtering programs can be fiddly. Spam Trainer provides easy (drag and drop) training from the Evolution mail client, and should work just as well with any email client that supports the drag and drop protocol implemented by the GTK+ toolkit.
Spamato is an extensible spam filter system. It is available as an add-on for Outlook, an extension for Thunderbird and Mozilla Mail, and a stand-alone proxy component. It provides a Bayesian filter, a rule-based filter, a Java implementation of Razor (Ephemeral and Whiplash), a filter that queries domains on Google, a collaborative domain, and a collaborative hash-based filter. Graphical statistics explain the efficiency of each filter as well as the overall effectiveness of the whole system.