- Import the old email into Thunderbird. (Other clients may also work.)
- Enable IMAP access to your Gmail account in Thunderbird.
- Drag and drop the old email folders into your Gmail account.
Previously, one could backup email into Gmail using a program like GMail Loader. A key advantage of the IMAP technique is that the original email dates are preserved in the Gmail database. With GMail Loader, the date when a message was imported shows up during search (though the original email date remains in the headers). Use of the import date messes up both search and Gmail's threading features.
Generally, importing email into Thunderbird is a pretty easy process. I successfully imported and backed up some old Eudora email and mail in the MH format (by converting to mbox format with the
packfcommand). The only downside of this whole process is the time one inevitably wastes reading ancient emails :).