You can use JavaMail to develop e-mail client applications.
The JavaMail API provides a platform-independent and protocol-independent framework you can use to build Java™ technology based e-mail client applications. You can use the JavaMail API to create a mail client capable of sending multimedia mail messages, as well as enabling a full fledged IMAP (Internet Mail Access Protocol) implementation supporting folders, authentication, and attachment handling.
Because SMTP only supports character data, it uses MIME to represent complex data such as formatted text, file attachments (text and binary), and multimedia content. If you use the iSeries™ QTMMSENDMAIL API, your application must take care of converting the data into the appropriate content. The JavaMail implementation provides MIME processing capabilities natively.
As of OS/400® V5R2, JavaMail components were included as part of the IBM® Developer Kit for Java.