MHonArc can process mail messages to create a set of indexed html pages.

Threaded and chronological indices can be generated, and messages can be added
incrementally to an existing archive.   A `standalone' mode is also supported
for quick conversions of individual messages to HTML without adding them to
the index.  MIME attachments in mail messages are handled.  MHTML is
also supported.

The program is highly customizable and is well documented.  See URL
for more information.