Migration of mail from PPC G5 iMac w/ Tiger to Intel iMac w/ Snow Leopard

Question

The migration was largely successful in that all my mailboxes from the old computer have appeared on the new one. The only thing I cannot do on the new iMac is delete some of the messages that have just arrived on the new iMac. They had NOT previously been downloaded to the old computer. The error message is this: the destination mailbox "(null)" does not allow messages to be moved to it.
I already have the mailbox behaviours set to delete messages from the server once I have downloaded them. I have also tried moving the mail.plist to the desktop, quitting Mail and then restarting it, both to no avail. Any help would be most welcome. Thank you.

Answer 1

I can move messages between mailboxes in On My Mac, and I have made an important, and helpful discovery. I have 4 active POP mail accounts, each obviously with its own e-mail address. In Snow Leopard, but not in Tiger, this seems to result in having 4 different sub Trash mailboxes, each of them corresponding to one of the 4 active mail accounts. By using the Move to command, as you suggested, I am able to "delete" an e-mail, PROVIDING I specify which of the 4 different Sub Trash mailboxes I wish to move it to. This was not necessary in Tiger as the Trash mailbox was a single integrated Trash mailbox and not compartmentalized so as to correspond to the 4 active mail accounts.
So I do now have a "work-around" that works, but having to select the Move to command on a drop-down menu and then select which of the 4 Trash mailboxes to delete the messages to is an incredibly long way round of deleting e-mail. Surely there must be a simpler way of deleting e-mail in Snow Leopard if one has more than one active mail account. Any more suggestions would be very welcome although I am about to go travelling for 10 days and won't have much access to the internet during that time.