This might help:
What do you want to happen if any email is invalid - throw error/message or ignore that one and continue?
if you want to stop if any are invalid, something like this might work (I have not specifically tested it!):
function formatter_checkemails($emailList){
$emails = explode(',', $emailList);
foreach ($emails as $email) {
$email = trim($email);
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
return false;
}
}
return true;
}