protected function assertMailCount($actualMail, $expectedCount = null)
    {
        $actualCount = count($actualMail);
        if (is_null($expectedCount)) {
            // If number to expect is not specified, expect more than zero.
            if ($actualCount === 0) {
                throw new \Exception("Expected some mail, but none found.");
            }
        } else {
            if ($expectedCount != $actualCount) {
                // Prepare a simple list of actual mail.
                $prettyActualMail = [];
                foreach ($actualMail as $singleActualMail) {
                    $prettyActualMail[] = [
                    'to' => $singleActualMail['to'],
                    'subject' => $singleActualMail['subject'],
                    ];
                }
                throw new \Exception(sprintf("Expected %s mail, but %s found:\n\n%s", $expectedCount, $actualCount, print_r($prettyActualMail, true)));
            }
        }
    }