I've changed ISPs and now have a new email address. In Apple mail I have not deleted the old account as I want to keep the contents.

I note that, if I go into Mail>Preferences>Accounts, I can enable the old account and see all the email. If I enable the new account, none of it is there…which I would expect.

I have not yet quit the old ISP and am wondering if I will be continue being able to see the old mail after I quit them. I think I should as I assume the mail is stored on my drive. However, I'd like to know for sure.

If the old account is POP then the messages are definitely stored on on your computer, if the account is IMAP maybe yes and maybe no. In either case the only rational way to access them rationally would be through Mail. To be on the safe side, before deleting the old account...

1. In Mail create folder or folders On My Mac
2. Drag the messages you want to keep from their current account to the folder(s) On My Mac

The messages On My Mac will be physically stored in folders on your Mac whether they are there now or not and they will be disconnected from the old email account server. Once you are satisfied the transfer is complete you can delete the old account without losing them. As a side benefit you will be able to access those messages without having to re-activate the old account.

Thanks joemike….the old account was POP, the new one is IMAP

After posting I did a bit of poking around in Mail and, under "File" I found "Import Mailboxes". That opened a dialogue box with a number of mail choices (Apple Mail, Netscape, Eudora, et cetera), so I picked Apple Mail.

Then it was a matter of navigating through Users to the Library and the Mail Accounts. The import process took quite a while (15 years of mail) but it imported all the mailboxes complete with all the right folders. Neat.

The only puzzler was that I had an extra folder duplicating all the same stuff that I had imported. I just deleted that folder and all is well.

All in all, a good day.

EDIT: It looks like I'll be using your suggestion for my wife's email. She has a separate account on our computer and, when I went there, I couldn't see the User Library.