4.7 Utils/Mail.php doesn't check if CC/BCC are null but should
Solution: replace current lines in Utils/Mail.php where no check for null CC, BCC is made, with the following (line 194 +) from older version of Mail.php:

if (CRM_Utils_Array::value('cc', $params) != NULL){
  $headers['Cc'] = CRM_Utils_Array::value('cc', $params);
}
if (CRM_Utils_Array::value('bcc', $params) != NULL){
  $headers['Bcc'] = CRM_Utils_Array::value('bcc', $params);
}


I don't know why the code was changed to not check for null. It seems it doesn't hurt to check, and if null CC/BCC are put into headers, mail() fails.