Putting the base mail directory on a shared partition (for different logins but same email) doesn't quite work. Mail received/sent from one login are not visible from another login. This is because the group and group rights on the new messages are empty. If I change them manually to be the same as the main Mail folder, it is OK. Can Claws use the base mail folder's group and group rights for new messages? In my case, the mail folder had group "homeusers" with rw rights.
Go to the folder Properties (right-click), and set 'Folder chmod' option to 664 on the 'General' page.