major problems moving/copying messages in Mail

I recently decided to move all the messages archived "On My Mac" in Mail to the MobileMe server. I innocently thought it would be as simple as just dragging the folders. Not so. It has turned into an epic struggle lasting weeks, and I have managed to lose and misplace a lot of messages in the process.

Now that almost everything is copied, though, I have some outstanding issues that I cannot resolve.

1. I cannot rename some of my folders. I get the following error:

The IMAP command “DELETE” failed with server error: Mailbox does not exist.

2. Some messages cannot be moved or copied due to the following error:

The IMAP command “APPEND” (to family) failed with server error: Message contains invalid header.

This is driving me mad! Basically I have a random collection of messages On My Mac that I cannot get onto the server, so it's a disorganized mess.

I have tried:

1. Rebuilding mailboxes

2. Exporting and reimporting the mailboxes containing the offending messages

No dice.

Any suggestions, please? Thanks in advance.