MailBee WebMail PHP is a free AJAX webmail application with skins support. Users can receive, view, manage, compose, and send email through web interface (POP3 and SMTP supported). Source code included.
MailBee WebMail Pro PHP is an advanced web-based email client written in PHP. AJAX interface, SMTP, POP3, IMAP, folders, rich-text editor, address book, multiple accounts per user, web-based admin panel, and more. Can use MS SQL or MySQL database.