wp-includes/PHPMailer/PHPMailer.php:4509Get the server hostname.
string protected function serverHostname() { $result = ''; if (!empty($this->Hostname)) { $result = $this->Hostname; } elseif (isset($_SERVER) && array_key_exists('SERVER_NAME', $_SERVER)) { $result = $_SERVER['SERVER_NAME']; } elseif (function_exists('gethostname') && gethostname() !== false) { $result = gethostname(); } elseif (php_uname('n') !== '') { $result = php_uname('n'); } if (!static::isValidHost($result)) { return 'localhost.localdomain'; } return $result; }Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/PHPMailer/PHPMailer.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.