What comes to my mind is that read messages are allowed to remain in the new mail folder and the moves are the result of a on-close new mail filter rule.
Check for a new mail filtering rule that is applied when the folder is closed, and check that the "Allow read messages to stay in the new mail folder" option is not enabled. This setting is in Tools > Options > General settings > Basic settings.
Hi Brian. I don't have any filtering rules and the Allow... option is not enabled. Also, a few of these emails actually do get moved each time I close the programme,
On starting, it sometimes says a folder is locked
