Error message

• Failed to load Mandrill PHP library. Please refer to the installation requirements.
• Warning: Cannot modify header information - headers already sent by (output started at /var/www/html/cfu/includes/bootstrap.inc:1640) in drupal_send_headers() (line 1499 of /var/www/html/cfu/includes/bootstrap.inc).
• Error: Class 'Mandrill_Error' not found in mandrill_sender_plain() (line 273 of /var/www/html/cfu/sites/all/modules/mandrill/mandrill.module).