Help with folders

[Zlat]

Dear Sir or Madam

Can anybody help me? Why are you using the '.mailbox' file?
I can't create a sub-folder to folder which already is a sub-folder too.
And when I want to create a sub-folder named 'ff' to the top-folder 'dd' I am taking the following message:
"The folder "/ff" was not created. This is what the server said: CREATE failed: Can't create mailbox node /ff: Permission denied"

What is it?
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Sincerely yours,

Victor Kowalenko (Zlat)
web developer and administrator.

E-mail: zlat@mail.od.ua
ICQ: 20072381
SMS: +380677304016