As mentioned in the servers.php file, the Horde Project does not officialy support this parameter. Use at your own risk.
For information on configuring UW-IMAP servers, see IMP UW-IMAP/Namespace Configuration Tips.
This documentation is also valid for MIMP 1.0.
Simple. The php imap_*() functions don't provide a way for us to get this information. Trust us, we would gladly use those functions if they existed.
Please see the documentation for the namespace function in imp/lib/IMAP/Client.php. Or you can try going to our API Documentation. (No guarantees this link will remain correct).
Thanks for asking. It looks like the following (this is an example of a dovecot-1.0 IMAP server):
'imap_config' => array(
    'children' => true,
    'namespace' => array(
        '' => array(
            'name' => '',
            'delimiter' => '.',
            'type' => 'personal',
            'hidden' => false
        ),
        '#foo.' => array(
            'name' => '#foo.',
            'delimiter' => '.',
            'type' => 'shared',
            'hidden' => false
        ),
        '#foo2.' => array(
            'name' => '#foo2.',
            'delimiter' => '.',
            'type' => 'public', (or 'other' if using IMP 4.2+)
            'hidden' => false
        )
    ),
    'search_charset' => array(
        'UTF-8' => true
    )
)
This is an example of an UW-IMAP server:
'imap_config' => array(
    'children' => false,
    'namespace' => array(
        'INBOX/' => array(
            'name' => 'INBOX/',
            'delimiter' => '/',
            'type' => 'personal',
            'hidden' => false,
        ),
    ),
    'search_charset' => array(
        'UTF-8' => true
    )
)
Another UW-IMAP example:
    'imap_config' => array(
        'children' => false,
        'namespace' => array(
            'mail/' => array(
                'name' => 'mail/',
                'delimiter' => '/',
                'type' => 'personal',
                'hidden' => false,
            ),
        ),
        'search_charset' => array(
            'UTF-8' => true
        )
    )
Obviously, don't copy this code directly into servers.php and expect it to work! This is merely an example. Your IMAP server configuration is almost certainly different.
Note: MIMP does not require the 'search_charset' value.
Note: As of IMP 4.2.1, the 'children' value is no longer needed.
imap_capability "IMAP4rev1 CHILDREN NAMESPACE"
    'imap_config' => array(
        'children' => true,
        'namespace' => array(
                'Mail/' => array(
                        'name' => 'Mail/',
                        'delimiter' => '/',
                        'type' => 'personal',
                        'hidden' => false
                ),
        ),
        'search_charset' => array(
                'UTF-8' => true
        )
    )
but using this config with dimp 1.1.1, when selecting a new message from the inbox, the preview would never show up and the "working" icon would never go away. if a message was not marked as new, it could be previewed without any problems. also preview of new messages under Mail/ folders worked fine as well.
To make the preview work for new messages in the INBOX i had to add a new namespace definition (maildir style):
    'namespace' => array(
            '' => array(
            'name' => '',
            'delimiter' => '.',
            'type' => 'personal',
            'hidden' => true
            ),
any maildir namespace will do to fix the issue. we dont want to support both formats so i am hiding this definition.