Invalid folder name

The mailbox name is typically a hierarchical name used to determin a folder on the server. So it is equivalent whith the folder structure you can see in your mail client.

There is no precise definition how the hierarchical name is formed. It depends on your mailserver implementation. Only the case-insensitive mailbox name INBOX is a special name reserved to mean "the primary mailbox for this user on this server".

But you should avoid using non-graphical characters, the list-wildcard cahracters ("%" and "*"), characters which are reserved by the server for the level of hierarchy (typically "/" or ".") as well as "#" as "&" because they can have special meanings.