test_support-1.0.x-dev/tests/modules/test_support_mail/test_support_mail.module

<?php

/** @param array<mixed> $message */
function test_support_mail_mail(string $key, array &$message): void
{
    if ($key !== 'test_support_mail') {
        return;
    }

    $message['subject'] = \Drupal::state()->get('test_support.mail_subject');
    $message['body'][] = \Drupal::state()->get('test_support.mail_body');
}