the plugin WP Mail SMTP v1.4.2 logged this error during the last time it tried to send an email:
Mailer: Default (none) PHPMailer was able to connect to SMTP server but failed while trying to send an email.
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Some issues is because php version of your server hosting not valid with your website. First, you must check what php version that your hosting used, check from WHM->server information (right menu).
Now, back to your cpanel dashboard, seleck multiPHP manager, choose your site and select php version(right menu dropdown), choose with the same php version of your server hosting.
Back to your WP dashboard, WP mail SMTP and test send email again.