If you’re getting The message … could not be moved to the mailbox … in Leopard’s Mail.app 3.0 or 3.1, you might want to uncheck “Move deleted messages to the Trash mailbox” in Preferences >Accounts > (account) > Mailbox Behaviors.
Thanks to James in this thread.
Hang in there gang. Together we can work out the bugs in the Mail.app 2 to 3 transition.
