Now on its 1.0 version after years in development, si.Mail is starting to look like a real light weight and stable alternative for a mail client.
It is readily portable -- i.e. can be used out of a USB pen drive, for instance -- and easily customizable -- although there's not much to choose from right now in the interface department. It includes anti-spam, mail database indexing and other advanced functionalities. Check it out at:
http://www.simail.si/
Screenshot