Musashi is an email client that supports multi-user, multi-user mail box sharing, drag & drop operation, and Japanese character sets on the appropriate OS versions. Background send and receive and searching and filtering are also supported.
Version 3.5 is now freeware. It appears that Musashi development has ceased, and the project is now in "archive" status on Sonosoft's Web site, but is at least still available for download as freeware.
Mac OS X 10.1 or later