When using a smart group as a mailing list, users who unsubscribe from the smart group are still included in the mailing.
It looks like in CRM/MailingBAO/Mailing.php, is not taking into account those who have been removed from the smart group. Civi version 5.3.
if (count($includeSmartGroupIDs)) {
$query = CRM_Utils_SQL_Select::from($contact)
->select("$contact.id as contact_id, $entityTable.id as $entityColumn")
->join($entityTable, " INNER JOIN $entityTable ON $entityTable.contact_id = $contact.id ")
->join('gc', " INNER JOIN civicrm_group_contact_cache gc ON $contact.id = gc.contact_id ")
->join('mg', " INNER JOIN civicrm_mailing_group mg ON gc.group_id = mg.entity_id AND mg.search_id IS NULL ")
->join('temp', " LEFT JOIN $excludeTempTablename temp ON $contact.id = temp.contact_id ")
->where('gc.group_id IN (#groups)')
->merge($criteria)
->replaceInto($includedTempTablename, array('contact_id', $entityColumn))
->param('#groups', $includeSmartGroupIDs)
->param('#mailingID', $mailingID)
->execute();
}