You can, of course, save the contents of a Mail mailbox as an archive. But the messages in those archives can only be read if you import the archive back into Mail. What if you want to archive those messages as an easy-to-read-and-search text file? Reader royhinckley has a solution that seems obvious, but has never been published on
When I wanted to archive e-mail messages as text, I spent nearly an hour looking through
Apple.com for an export menu, script, or third-party software that would do it for me. Then I found the too-obvious solution: select the group of messages you want to archive (no need to expand threads) and choose Save As. At the bottom of the Save As window, make sure the Format pop-up is set to Rich Text. Mail will then save the selected messages (expanding threads as necessary) to a single RTF file, complete with images and other attachments.
Another way to do much the same thing: Select the messages, copy them with Command-C, open a new RTF document in TextEdit, and paste the messages with Command-V. Also, as Hints reader sudo points out, in Mail you can also save selected messages as Raw Message Source (instead of Rich Text), producing Unix-friendly mbox text files.
Updated at 10:55 PT to correct Control/Command key mix-up.