Migrating from Eudora to Mail

Eudora is a mail program from way back, which many people adhered to because of its relative simplicity and stability. But with MacOS X moving on, this veritable program no longer runs. However, its mailboxes can be imported into more modern programs, such as e.g. Mac Mail. A few precautions need taking however.

In IN mailbox from Eudora was formulated as a single text file containing all the messages. Over the years, this file could grow to a very large size indeed. Because of its size, it is quite possible that it had become corrupted. Not show stopping as far as Eudora was concerned, but perhaps sufficiently so to prevent reliable import into another program. So it's worth probably ensuring this mailbox is as healthy as one can make it. Fortunately, all you need is a text editor, and some courage. Follow these instructions.
Armed with a healthy Eudora mailbox file, you can now import into e.g. Mac Mail (not sure if one can do this for e.g. Mac Outlook). One should not expect any issue with small mailboxes, but if the Eudora IN file exceeds say 100 Mbyte, it might be rather slow. Anything much larger is somewhat unknown territory (it always was a good idea to rename the IN mailbox every year or so, and let Eudora generate a new IN box. That way, it never would have gotten excessively large).
Mac Mail does not so much import a mailbox file, as all such files located in a specified folder. So its best to create such a folder for the purpose.