When last we met, I offered a somewhat convoluted method for archiving Mail 2.0 .emlx messages and importing them into another email client. Thanks to a suggestion from Macworld forum visitor ericob, I can offer an easier path.
That easier path is John Seward’s $30 MailSteward, a utility that imports messages from Apple’s Mail and places them (and their attachments) in a sortable database. The top half of the MailSteward window provides a list of your archived email and the pane below displays the contents of the selected message.
From MailSteward you can then export the archive as a tab-delimited text file, database file, mbox file, or SQL file, allowing you to import the archive into a database application such as FileMaker or into an email application other than Mail. Additionally, you can select a message in the database and click Reply to This Email and your default email client will launch with a message addressed to the sender of the selected message in MailSteward.