I just updated to the latest version of the MIME email class, and smtp.php is no longer present. It appears other files are missing too. I don't see these changes referenced in the changes here: https://www.phpclasses.org/browse/feed/changes/package/9.rss
If you use PHP Composer tool, you can install those packages automatically. The page above has the configuration lines for PHP composer configuration file composer.json . Just click on the Install with composer button after you are logged in the PHP Classes site.
Can you try these instructions and reply here to let me know if it works for you?