If you're sure that your mail server works fine, something is wrong with the configuration of Mibew Messenger and/or with the accessibility of your mail server from your web server. One should check the logs of both servers for any errors.
I also tried editing the files vendor/swiftmailer/swiftmailer/lib/classes/Swift/SmtpTransport.php
It's almost always a bad idea to mess with an included vendor software. Unless it's absolutely necessary and you're really know what you're doing.
/configs/default_configs.yml
It's useless. That file is just an example of configuration. Mibew Messenger doesn't use it.