moving [lots of] messages between mailboxes and folders in mail

i have maybe 500,000 messages in an imported mailbox on a new mac and i want to put them in my principal inbox. i can drag and drop about a days messages successfully (maybe 150) but when i try to take them all or even a years messages (maybe 40,000), when i drag the selected message i get a little icon next to the cursor that gives me the number of messages, the target inbox lights up and i drop the messages and the disappear from the imported mailbox but they never show up in the target mailbox and then a few minutes later they are back in their original location.

i don't know why you can't chose the folder or mailbox that is a target for your import in the first place which would eliminate this step (hello out there) which is the obvious need when moving from one computer to another. but i guess that's a question for the geniuses at the mother ship.

Is there a known limit or why is this happening? or is there another way to migrate these messages?

(and i'll search and/or put up another question about syncing because it is taking me so long to deal with this migration that i have multiple back-ups to import and there is overlap and gaps and i now need to sync the mailboxes between multiple machines without creating duplicates . . .)

thanks,

brian