Due to reasons explained below, I’m creating a fresh user account on my MacBook Air M2 running Ventura 13.6.4 (22G513). I use Mail version 16.0 (3731.700.6.1.1) as my primary memory and communication hub. 95% of my mail is IMAP via Pobox; I also have IMAP iCloud and Outlook addresses. I’ve got around 25,000 emails I’ve saved on my Mac, some of which date to 1995, and I do refer to them often. (They’re on my Mac because they span four previous POP email addresses.)
What’s the best way to copy mail from my previous account to the new account?
Details: I’ve been using Migration Assistant since at least Mountain Lion. Mail crashes daily and my Smart Mailboxes are no longer working. Two vital Safari extensions are displaying bugs that mystify their creators. Since the Safari extensions work fine in a test account, I decided that time spent on Spotlight reindexing might as well be devoted to a fresh start.
My Mailsteward archives go back to the early 2000s. Once in a while I copy the archive to a Bluray disk (50GB) for long-term storage. Recently I searched that database and managed to restore some lost emails to an mbox folder, although Apple Mail refused to open it and I have to use another app (see the 29th post in the above link).
Regarding your question, I suspect there will be difficulty moving 25,000 emails from various email accounts to a fresh user account on your Macbook Air. The recent email accounts like iCloud should be okay to deal with as you just log into the email account as a new Mac user and the emails should copy over from the cloud.
However emails that are not cloud-based will, I understand, not be easy to copy. From my experience these mailboxes seem to be encryted and tied to the current Mac user account and so the mbox folders cannot be copied and opened with another Mac user account.
Others may have tips to do this.