Graham. has brought this to us :
But don't you have a suite of folders (inbox, outbox, sent items,
etc) for EACH mail account?

No, I have it sorting incoming mail into folders based on the subject
line. 10 folders in total, but just one outbox and the outbox is empty.
A bit of reading suggests these stuck emails are read receipts,
probably automatic read receipts, which do not appear in the outbox.
Mail which came in via Tiscali, but read receipts would not have been
able to be sent out by Tiscali servers, because they do not allow it.