Major Description
Each SelectedMailbox has some msn<->uid mapping in cache to make it quick to convert between the two. At the moment we throw an MailboxException if we can not get the uid or msn for the given uid/msn. This is wrong in a few cases as it may happen that another concurrent session deleted the message with the uid/msn in the meantime while we create our response. In this case we should just skip it.