mbox and mail folder

[Chuck Peters]

I thought I would find this in the FAQ. A user reported not being able to
create folders via imp, for example when sending mail via imp, an error
occurs saying "Message sent successfully, ERROR: not saved to
mail/sent_mail."  Other mail clients the user has are Outlook and Pine.  I
thought this would be a permissions problem, but that had no effect.  If I
move or remove the mbox file from the users directory, she can create mail
folders.

These are all Debian boxes.
Mail server
ii  imap           4.7c-1         remote mail folder access server for Pine an

Webmail
ii  imp            2.2.3-0.potato Web Based IMAP Mail Program.

What is the solution to our problem?

Thanks,
Chuck