calypso2mbox is a script written in Perl that will allow you to migrate away from the Calypso email program. Calypso is a now defunct email program for Windows. It also works with Courier email 3.5 as of version 1.1. Calypso had an export feature but it didn't properly preserve the HTML parts of messages or the attachments - it just dumped them in a folder and mangled the file names while concatenating all the text part of the message into an mbox-like file. In short it was useless unless you didn't mind losing all the attachments and HTML emails you'd received over the years.
As part of moving my home computer to Linux I wanted to migrate my email away from Calypso and use a Linux email client. Since I couldn't find any other way of doing this I wrote this script. After a lot of tweaking it successfully transfered a few thousand messages from different folders into a properly formatted mbox file containing properly formatted mime multipart emails. I successfuly used the files created by calypso2mbox with Evolution and later Kontact. I also dumped them in my ~/Mail directory and they were successfuly picked up by my IMAP server (wu-imap). These files may also be importable into some Windows email clients, I'd like to be more helpful here but I have never needed to try it. If you have success with a particular client email it to me and I'll add a note here.
13/3/2007 - Thanks to Clemencio Mascarina, Jr. (clemmm AT gmail dDOT com) for some updates to get it working with Courier email 3.5. These updates have been released as version 1.1.
Download: calypso2mbox 1.1
You may also need ActivePerl Standard Edition if you plan to run it on Windows.
See the calypso2mbox SourceForge Project page for more info.