How do I fully export emails from Mac mini running macOS Sequoia 15.2 before changing internet providers?

Re: "I'm changing internet providers and want to ensure access to all my emails after I'm disconnected

from my current service (ATT/Yahoo).

I tried exporting my inbox and various archives to my hard drive but it makes only a partial copy"

Wondering what method of exporting you tried:, and if you tried exporting in smaller batches

See Apple Support's article for MacOS Sequoia: Import or export mailboxes in Mail on Mac - Apple Support

Eg: says:

"Export mailboxes

1. Go to the Mail app on your Mac.
2. Select one or more mailboxes, then choose Mailbox > Export Mailbox.
3. Choose a folder or create a new folder, then click Choose.
4. Mail exports the mailboxes as .mbox packages. If you previously exported a mailbox, Mail doesn’t overwrite the existing .mbox file; it creates a new .mbox file, such as My Mailbox 3.mbox."

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Re: "Is Exporting the best method for what I'm trying to do?"

✽ Am no expert in this area : Seems to me exporting is a good way (not the only way) to go,

Best method is an individual opinion for each person's circumstances.

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Re: "If yes, why doesn't it work?"

✽ Sorry. Could be any one of various possible causes

... can not guess from my own inexpert experience, or from the info. provided :

(Maybe try exporting in smaller batches ?)

Eg: We don't know what limitations, if any, your email provider's service may have in place?

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Re: "If no, what process should I use instead?"

✽ Again, it's a matter of opinion: Eg: You can: Save emails as files or PDFs in Mail on Mac - Apple Support

✽ In the Mail app: We have the option to save eMails "On my Mac", or keep them on the eMail providers site

Eg: "On My Mac: Mailboxes you create in On My Mac are local,

meaning you can access them only on the Mac where you created them."

from: Create or delete mailboxes in Mail on Mac - Apple Support

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Useful resource: Mail User Guide for Mac - Apple Support

Thank you for your prompt reply and your ideas.

Re: "It looks like you stumbled onto a workable plan: create a new inbox on my Mac and manually copy the emails from the server inbox into the new, local inbox."

As I keep a large number of mailboxes "On my Mac", mostly created as needed, it makes any email much easier to find, re-view, or use. We can also have mailboxes within mailboxes as with any folders ... so, is hard to imagine how many inbox emails you have to keep secure. ( I endeavour to keep server-inbox empty, ie: is ready for more emails, and easier to deal with the daily flow.)

Re: "It'll still take a long time since Mail seems to choke on moving more than 10-15 emails in a group but it seems like that's the only path to success."

To move a large number of emails to a newly created or existing mailbox:

Try clicking the first email in the list to highlight it, then hold down the shift key while you click on the bottom email in the list (or, if a huge number of emails, you might need a reasonable number to work with and move in one go) all the emails between the two you clicked should highlight as selected. Then: You can likely hold down the shift key once again, while you drag all those selected emails to the new mailbox you created, or any existed mailbox (Archive?).