mail in IMAP folders to local folders

Hi weisen.

1. What's the proper way to copy IMAP folders to local folders?

What you did is correct and should have worked. Another way to do it would be create the destination mailboxes one by one and copy the messages from each IMAP mailbox to the corresponding “On My Mac” mailbox, one mailbox at a time — by selecting the messages and either option-dragging the messages themselves or doing Message > Copy To.

2. Is it possible to have a local "On this mac" folder with messages
that are somehow still linked to/dependent on an IMAP server?

No.

3. Is there a real per-message and per-folder Get Info or Reveal in Finder
that can tell me where a particular resource is located?

No, but Mail stores all your mail within the ~/Library/Mail/ folder. Cached IMAP mailboxes are located in a corresponding ~/Library/Mail/IMAP-username@mailserver/ account folder. Custom “On My Mac” mailboxes are located in ~/Library/Mail/Mailboxes/.

As long as you copy the messages and don’t just move them, you may try again and see what happens. In particular, check in the Finder whether the “On My Mac” mailboxes have as many .emlx files within the respective Messages folders as messages you’d expect to be there.

Also, instead of rebuilding the mailboxes, you may try re-creating the whole index, as follows:

1. Quit Mail if it’s running.

2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is locally stored.

3. Locate Envelope Index and move it to the Trash. If you see any other “Envelope Index”-named file there, delete it as well.

4. Move any “IMAP-”, “Mac-”, or “Exchange-” account folders to the Trash. Note that you can do this with IMAP-type accounts because they store mail on the server and Mail can easily re-create them. DON’T trash any “POP-” account folders, as that would cause all the mail stored there to be lost.

5. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index — Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.

6. As a side effect of having removed the IMAP account folders, those accounts may be in an “offline” state now. Do Mailbox > Go Online to bring them back online.

7. As a side effect of re-creating the index, some previously read messages may become unread again. This is “normal” (and may also happen when rebuilding individual mailboxes by means of Mailbox > Rebuild). You may fix that with Message > Mark > As Read.

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e. /Users/username/Library.