One of the problems of using four operating system old software is finding outdated versions of software. EmailArchiver may be the go, and they do say you can get versions back to OS X.6, but I don't see that on the web site, just S X.10-OS X.8 version. You may have to ask them and good luck with it:-
Email Archiver
You can archive your email for free by using the export feature built into mail. Go to "Mailbox" and select "Export Mailbox". That will allow you to save it elsewhere or to an archive folder of your choice. You can also export individual mail messages as PDF but that might be a tedious job if you have many messages to do.
The program Harry recommended is worth the $24.99 if you have large amounts of email to archive and plan on using it again for other archiving jobs.
What do you mean "export feature built into mail"? Are you talking about Google gmail app for educators?
Go with mailstore which is the mail archiving solution.