From Stephan Schulmeister Software Development:
eml-2-emlx is a mail file conversion utility. It convertes the standard eml-files to emlx-files and .mbox mailboxes. eml is a standard mail file format often used by e-mail programs. emlx is the file format used by apple in MacOS Mail v2 and above. eml-2-emlx does not just convert mails, it preserves your mailbox structure to get it into Mac OS Mail. Using Mac OS X Lion (10.7) and above you can now directly convert to a Mac OS X Mail Structure, which you can import with Mac SO Mail using the import Apple Mail.