Note: You are currently viewing documentation for Moodle 1.9. Up-to-date documentation for the latest stable version is available here: error/mnet/auth imapnotinstalled.
This error indicates that there is an error when trying to call imap_open PHP function, that is because you need to configure PHP with the option --with-imap.
For ubuntu user, installing php5-imap may fix the problem
sudo apt-get install php5-imap
For more information visit http://php.net/imap