I’m sort of a packrat when it comes to email messages, so by the end of the year my IMAP account is cluttered messages I’ve read. I like to keep them around for reference, but I also want to clear out that space on my mail server. Here’s my process to accomplish both. (I generally wait until mid-year to do this, to help ensure I no longer need instant access to old messages.)
In the macOS Mail app, create a Smart Mailbox that gathers all messages for the year. Be sure to include Sent messages so you have a complete record.
When the Smart Mailbox has finished finding all the messages in the date range, create a new regular mailbox (not another smart one). Select all the messages in the Smart Mailbox and drag them to the new non-smart mailbox. (It feels like you shouldn’t need to do this, but trust me, you do.)
Wait patiently while the Mail app moves all those messages over to the new mailbox. This will take a while, and if you squint, you can monitor the progress at the bottom of the Mailboxes pane.
When Mail has finished moving the messages to the mailbox, Command-click the mailbox and choose Export Mailbox from the pop-up menu. Choose a destination (I always make a new enclosed folder) and let ‘er rip. This will take a long time to finish. Once again, watch the progress indicator to see if it’s done.
When it is finished, you’ll have an “mbox” folder saved at the destination. At this point, your messages are safely archived on disk and you can go back to Mail and delete both the mailboxes you created during this procedure. Again, keep an eye on the progress indicator. Removing all those message from the server will take time. But you’ll be left with a nice uncluttered IMAP account.
Now, how do you look at or search the archived messages? I mean, that’s why you kept them, so you can refer to them, right? The files on disk are just text, so something like HoudahSpot, or even the Finder (gasp), can search them. But you’ll be happier if you use an app that understands the mbox format and presents them more or less like how you expect email messages to appear. EagleFiler would be a good choice, I hear, but I don’t use it myself. I am already a heavy DEVONthink user, so that’s my app of choice. I import each mbox into a database that’s dedicated to my old email.
If you do this, notice that DEVONthink has (unfortunately) taken a cue from Mail and shows its own barely noticeable progress indicator. You’ll want to make sure importing is finished before quitting DEVONthink and throwing about the mbox files.