Transferring the contents of Mail from an old iMac to a new one

> The account is indeed a POP account

Oh, that's unfortunate.

As indicated in my earlier post, POP3 mail is downloaded to your Mac, not kept server-side, so you're going to have to do the work to move the mailboxes over.

It's a three-step process. Export the mail from the old Mac, transfer the output file, then import into the new Mac.

You'll need to repeat this for each account and, potentially, each mail folder (if you have the mail broken out into multiple folders).

First, on the old Mac, Ctrl-click on the mailbox in the sidebar and choose Export Mailbox...

Give the export file a name and let Mail export the data.

Transfer this file to the new Mac (File sharing, iCloud sync, AirDrop... any way you like)

On the new Mac, open Mail and choose File -> Import Mailbox. Choose the format (Apple Mail, in this case)

The messages will be imported into a new folder, so you may have some work to do to re-align to your normal workflow.

(note: it's technically possible to use file sharing to mount the old Mac's drive on the new Mac and import directly from ~/Library/Mail but if you have multiple accounts it can be hard to track which accounts/folders you're dealing with. By first exporting the messages you want, you know what you're getting).

As an aside - any reason you're using POP3 for this? Most services have moved away from POP3 in favor of IMAP

What Mail account?

If iCloud, then you don't need to do anything - all iCloud mail is stored on the mail server, so when you connect your new Mac Mini to iCloud, it should see all the messages.

Many other mail services work the same way - Gmail, Outlook/hotmail.

The exceptions to this are:

a) older accounts that are configured to use POP3 (Check Mail -> Settings -> Accounts to see if you have any POP3 accounts listed)

b) folders that you have setup On My Mac (Check the sidebar in Mail.app). As its name suggests, these are folders of messages on your Mac itself, not on the server. If you have any there, there may be hoops you need to jump through to get these messages over, but most people don't, in my experience.