Loading email templates in a Craft 3 plugin
I think you have to set the template mode to 'SITE', to let Craft know where to look for the template:

$view = \Craft::$app-&gt;getView();         
if ($view-&gt;getTemplateMode() !== $view::TEMPLATE_MODE_SITE) {
    $view-&gt;setTemplateMode($view::TEMPLATE_MODE_SITE);
}

Craft::$app-&gt;mailer-&gt;compose('_email.html', ['body' =&gt; $emailBody])
    -&gt;setFrom($fromMail)
    -&gt;setTo($toMail)
    -&gt;setSubject($subject)
    -&gt;send();