Archieving emails in mass

Here is what I do with all of my messages:

1) Create a new mailbox "On my Mac".

2) Move those e-mails to the new mailbox. I strongly recommend doing this in small batches of a few thousand at a time. Consider organizing them by year. Create a new mailbox or folder for each year and move each year's messages to its designated folder.

3) Export the data in "mbox" format.

At this point, you can re-import those messages on a new machine in the future. You won't retrieve all of them. I've found that I lose a few messages per thousand.

You can leave the messages in your "On my Mac" area if you want. They will no longer be cluttering up and slowing down your online accounts. If you want to search them, you will need to manually select the mailbox.

Note that Apple Mail has very poor progress feedback. You will need to use Activity Monitor to check the progress of each batch move. While Mail is moving messages, it will show a relatively high level of CPU usage. When that settles down, you can move the next batch. This may take some time, perhaps a few days. It's tedious.

The same goes for the export. You can look at the files on disk during the export. The mbox name will be something like "whatever.mbox.partial" until the export completes.