One of my customer can’t add a maibox because this mailbox reportedly exists, but it’s not in the control panel.

If you don’t see a mailbox in the CP but it exists on the mail server you can do the following:
1. Log in your mail server as root.
2. Change directory to ~vpopmail/bin/
3. Check if this mailbox exists:
# ./vuserinfo box@domain.com
4. If it exists and you don’t see it in the Control Panel interface, you can try to delete it:
# ./vdeluser box@domain.com
5. Try to create this mailbox through H-Sphere Control Panel.