I have 2 e-mail accounts set up in Apple Mail. I have added some extra Mailboxes in one account so that I can sort the important messages that I want to keep. At first these seemed to work OK, but in the last few days they just vanish in an apparently random fashion when waking the iMac. I can get them back by Synchronising All, but all the messages have to be downloaded again from the server. This is moderately annoying. Is there an explanation, and what can I do to prevent this odd behaviour?