in wp-includes/formatting.php, function sanitize_option(), case 'admin_email', I made the following change:
case 'admin_email':
$values = explode(",", $value);
foreach($values as $k=>$v)
$values[$k] = sanitize_email($v);
$value = implode(",", $values);
break;
then, in wp-includes/pluggable.php, function wp_mail, I made the following change:
function wp_mail( $targets, $subject, $message, $headers = '' )
{
$tars = explode(",", $targets);
foreach($tars as $to)
{
// original function code here except for last return line
}
return $result
}
This allows commas between emails in the admin email setting, and ONLY the admin email setting, and the wp_mail function will mail whatever to every email separated by a comma. But, since as stated the admin_email is the only one we allow commas for, this is the only one that can receive this multiple-email functionality. We only return the $result of the attempt to send the last email. I don't think that will cause any problems though.
This particular method requires a comma, and a comma only, between the email addresses. However, you could change the explode/implode functions to allow whatever delimiter you desire, just make sure to make it something that isn't valid in email addresses anyway, so you don't split up someone's valid email address.