The EagleFiler manual/help says the following, under Importing Mail from Apple Mail > Missing Messages and Attachments:
When importing from Apple Mail, EagleFiler may report a Missing Apple Mail Messages or Missing Apple Mail Attachment error. This indicates that the message had not been fully downloaded to your Mac. You should tell Mail to download the messages and then try the import again. There are several ways to do this:
- For very large numbers of messages, the [Download Apple Mail Messages] script can take a while. A faster option is to Option-drag and drop the entire mailbox to the On My Mac section of Mail’s mailbox list. This will create a local copy of the mailbox with all the messages fully downloaded.
In Mail 12.4 (3445.104.17), it looks like dragging and dropping IMAP mailboxes to On My Mac defaults to copying—that is, the cursor displays the plus-sign-in-a-green-circle icon without Option held down. Does Option-less dragging-and-dropping also cause Mail to fully download messages, or is Option somehow critical to that?
(I assume that copying mailboxes and not messages is all that matters here, but Mail seems so finicky about this that I want to make absolutely sure.)