Sylpheed
Syplpheed is a lightweight GTK+ mail/newsgroup client. It runs on Unix-like systems such as Linux, BSD, and Mac OS X. It also supports Windows.
It has the required libs bundled and stores all info in Documents and Settings\User\Application Data\Sylpheed under Windows. It has multilanguage support and can be migrated to from any mail client that uses mbox.
I like this client very much, I dropped Thunderbird for it, and since I travel a lot, I would like to request it to be a portable app.