Taitava\SentEmails\ImapSentFolderFactory::include_plugin_classes
SilverStripe doeesn't autoload taitava/swiftmailer-imapsentfolder because it doesn't contain a _config folder nor a _config.php file. I don't know a better way to include these PHP classes, so do it just with the basic require_once command.
Signature
private function include_plugin_classes()