Sylpheed

From Grokdoc

sylpheed is a mail GUI written using the GTK system. It co-exists with the MH mailsystem, using a representation of one file per message, one directory per folder.

sylpheed implements its own optimised indexing system externally, which is not recognized by MH.

sylpheed has advanced email filtering options which can be used to implement spam filtering and other mail sorting/collating functions.

A port to Microsoft Windows also exists.