Mail.app in Leopard has issues with moving mail with Zimbra.
This is a complicated issue, as the problem does not appear if using other IMAP servers, and does not appear with Zimbra using other clients - therefore there is some weird combination.
This issue is this:
- Move a message from Folder A to Folder B
- Now look through Mail.app - Folder A no longer has the message
- Still in Mail.app - Folder B now has the moved message
- Now in Zimbra Web client - Folder A has the message
- AND Folder B has the message
- And the message in Folder A is still marked Unread.
Initially I thought this might be something to do with Inbox - as it is sort of special. So I started moving all my incoming mail to a normal folder, and then moving the messages from there - well same problem.
Next time you run Mail.app it downloads and reads all the messages in that folder again - sometimes however it will skip them - somehow it knows they have been dealt with. Most of the time though they are loaded as brand new messages.
This means you CAN NOT USE Mail.app - why because reading, moving, deleting mail, is all basically ignored.
Because this does not appear with Mail.app with other servers - I have documented it here.
There is also many other references to this on the internet.