Error message

• Warning: Cannot modify header information - headers already sent by (output started at /home/hnkm7198/public_html/subdom/pbi.hnk.my.id/includes/common.inc:2861) in drupal_send_headers() (line 1554 of includes/bootstrap.inc).
• Error: Call to undefined function mail() in DefaultMailSystem->mail() (line 84 of modules/system/system.mail.inc).