OK, I have a huge "Import" group of mailboxes on the new iMac. Sucked the things off the old iMac. Fair enough, but there are now two folders inside each 'imported' mailbox; one contains the actual messages (even though they seem to also be in the particular 'imported' mailbox) and an "Attachments" folder, containing...'stuff!'
Now, there seem to be two different ways to put these old messages into new locations with the same first step: Recreate a mailbox with the whatever name (although usually the same one as the 'imported' on). You can now drag the messages out of the old mailbox and into the newly created one...over and over and over as one goes through all the boxes.
I soon tired of all this dragging and dropping and found one could simply drag the "Messages" folder into the list and then just rename it to whatever it should be. As a matter of fact, one could just drag the 'imported' mailbox, still with its sometimes two sub-folders to a new location.
The problem remaining, however, is where to put those 'Attachments' in Lion? I'd check a with a test but I seem to create a very time-consuming "Updating Cache Directory" and "[MobileMe] Communicating with server" activity. I may not have Mail setup correctly, yet...
OK, found everything that's arrived today in my Junk folder...Spamsieve is still not running with its previous 'corpose,' just another item to find on the old machine.
Of course, dragging all the files out of Junk and into the Inbox is causing another humongous thrashing of adding/caching/checking/updating/etc. If/when it ever stops, I'm going to restart to see if Spamsieve will cooperate. Right now, I gotta leave for a few hours...later!