Yes, the permissions have been set appropriately. When I migrated between servers earlier this summer, I had no problems transferring messages between user accounts... but in that case, the folders were being copied directly.
In this case, I need to copy large batches of messages within an account to separate folders, and be able to still view them in webmail (by selecting the appropriate folder). New folders work ok. Messages within the folder do not. I'll keep trying.
Thanks for your help.
-- Matt