[imp] Horde folders and attachment size

[Andy]

red hat 7.3
qmail-1.03
courier-imap
horde-2.1
imp-3.1

Problem about folder in horde:

I want to create a top level folder whose directory level is the same as
INBOX but I couldn't, an error "The folder "test1" was not created. This is
what the server said: Invalid mailbox name", what should I need to
configure?

Moreover, the attachment size of my horde is limited to 2 MB, I think it is
the default value, can I alter the size of attachment for my users?

Thanks for help!

Andy