After scouring Google for several days looking for a solution to this common question, and after subtle prodding on the Imp mailing list, I decided to roll my own method of making IMP create the default sent, trash, drafts, and spam folders as defined in "horde/imp/config/prefs.php" the first time a user logs in. This code was written for IMP 4.1.x.

What we need to do is create a Maintenance Task in IMP similar to the default "tos_agreement" task. The first thing we need to do is create a maintenance task. This is done by creating a PHP script called "horde/imp/lib/Maintenance/Tasks/create_default_folders.php" which contains the following class:

<?php

class Maintenance_Task_create_default_folders extends Maintenance_Task {

    function doMaintenance() {

        global $prefs;

        require_once IMP_BASE . '/lib/Folder.php';

        $imp_folder = &IMP_Folder::singleton();

        $folder_options = array(

            'sent_mail_folder', 'drafts_folder', 'trash_folder', 'spam_folder'

        );

        foreach ($folder_options as $this_folder) {

            $folder = $prefs->getValue($this_folder, true);

            if ($folder) {

                $folder = IMP::folderPref($folder, true);

                if (!$imp_folder->exists($folder)) {

                    $imp_folder->create($folder, true);

                }

            }

        }

        return true;

    }

    function describeMaintenance() {

        return _("This process makes sure the default folders are created on your account.");

    }

}

Now you'll need to edit "horde/imp/lib/Maintenance/imp.php" to reflect this change:

     var $maint_tasks = array(

        'tos_agreement'              => MAINTENANCE_FIRST_LOGIN,

        'create_default_folders'     => MAINTENANCE_FIRST_LOGIN,

// If you have users who often "accidently" delete their folders, you can do this instead:

//      'create_default_folders'     => MAINTENANCE_EVERY,

        'fetchmail_login'            => MAINTENANCE_EVERY,

        'rename_sentmail_monthly'    => MAINTENANCE_MONTHLY,

        'delete_sentmail_monthly'    => MAINTENANCE_MONTHLY,

        'delete_attachments_monthly' => MAINTENANCE_MONTHLY,

        'purge_trash'                => MAINTENANCE_MONTHLY

    );

Finally, add a preference in "horde/imp/config/prefs.php" like this:

    $_prefs['create_default_folders'] = array(

        'value' => 1,

        'locked' => false,

        'shared' => false,

        'type' => 'implicit');