Freron Software has introduced MailMate 1.0 (http://freron.com), an email client for Mac OS X 10.5 and higher.
MailMate is designed for and only supports IMAP while still working when offline. It features searching capabilities and smart mailboxes. It also has alternative message viewer layouts and a browser-inspired design.
According to the folks at Freron Software, the encouraged workflow in MailMate is to handle each incoming email by either archiving or deleting it. Smart mailboxes can then be used to automatically organize emails based on any message header of any message body part.
MailMate includes a set of alternative layouts for the main view. For example, a widescreen layout with the message view to the right of the messages outline and a correspondence layout which allows one to immediately see any messages in any existing email correspondence related to the currently selected message(s).
MailMate 1.0 costs US$. A demo is available for download.