Unable to delete messages - a Mail.app 3.0 bug

There's a bug (surprise!) that hits a few Mail.app users when they upgrade from OS X 10.4 to 10.5.

I just ran into it upgrading my mother's ultra-plain Mac Mini from 10.4.11 to 10.5.2 and then directly to 10.5.7.

I did an update in place, not an archive and install. I didn't run Mail until the machine was at 10.5.7 and had gone through a post-update "safe boot" cycle to clean out old caches and debris. So I was surprised to find a very significant bug!

When I tried to delete an email from Mail.app 3.0 I got this error message:

The destination mailbox “Deleted Messages ... " does not allow messages to be moved to it.

I found several posts on this topic, but they were all IMAP configurations. The associated fixes in those posts didn't work.

My mother's mail uses POP against her Gmail account, this 2008 post by "Davl" had the fix:

In finder I opened the folder containing the mailbox folders of my POP account. It was located at:

My User Home Folder --> Library --> Mail --> POP/account name --> additional POP folder

In this folder were the mailboxes folders with .mbox extensions.

• Deleted Messages --> empty
• Drafts.mbox --> Messages
• INBOX.mbox --> Messages
• Sent Messages.mbox --> Messages

Note my Deleted Messages doesn't have a .mbox extension like the others do...

This is what I found  in icon view. Clearly there's something wrong with Deleted Message!

Yech.

This is what I did after confirming my Gmail POP settings were correct.

• In Mail.app I saw some "child" trash items. I deleted those and emptied the Trash.
• I quite Mail.app and moved the "Deleted Messages" folder to the desktop
• I restarted Mail. app and then deleted a message. The Deleted Messages folder was recreated

This bug goes back over a year, so it's disappointing that even in 10.5.7 Apple hasn't fixed it.