Gmail API - send email to external recipients - gmail-api

Question

I've developed an app (PHP) to send email from my own web service using the Gmail API. We have Gmail / Google Workspace for the company. It is possible to send emails to internal recipients.
However, I can not send emails to external recipients.
I use a service user for the authentication.
I have the user in TEST MODE.
What could I have missed?
require_once __DIR__.'/vendor/autoload.php';
function sendmail($mailto, $from, $cc, $subject, $message, $file) {
$cc = null;
$uid = md5(uniqid(time()));
$client = new Google_Client();
$client->setAuthConfig(__DIR__ . "/credentials.json");
$client->setScopes(['https://mail.google.com']);
$client->setSubject($from);
$service = new Google_Service_Gmail($client);
$header = "From: ".$from." <".$from.">\r\n";
$header .= "Reply-To: ".$from."\r\n";
$header .= "To: ".$mailto." <".$mailto."> \r\n";
if(!is_null($cc)){ $header .= "Cc: ".$cc." <".$cc."> \r\n";}
$header .= 'Subject: =?utf-8?B?' . base64_encode($subject) . "?=\r\n";
$header .= "MIME-Version: 1.0\r\n";
$header .= "Content-Type: multipart/mixed; boundary=\"".$uid."\"\r\n\r\n";
$header .= "This is a multi-part message in MIME format.\r\n";
$header .= "--".$uid."\r\n";
$header .= "Content-type:text/html; charset=iso-8859-1\r\n";
$header .= "Content-Transfer-Encoding: 7bit\r\n\r\n";
$header .= $message."\r\n\r\n";
if(is_file($file)){
$filename = basename($file);
$file_size = filesize($file);
$handle = fopen($file, "r");
$content = fread($handle, $file_size);
fclose($handle);
$content = chunk_split(base64_encode($content));
$header .= "--".$uid."\r\n";
$header .= "Content-Type: application/pdf; name=\"".$filename."\"\r\n"; // use different content types here
$header .= "Content-Description:\"".$filename."\"\r\n"; // use different content types here
$header .= "Content-Transfer-Encoding: base64\r\n";
$header .= "Content-Disposition: attachment; filename=\"".$filename."\"\r\n\r\n";
$header .= $content."\r\n\r\n";
}
$header .= "--".$uid."--";
$mime = rtrim(strtr(base64_encode($header), '+/', '-_'), '=');
$msg = new Google_Service_Gmail_Message();
$msg->setRaw($mime);
$service->users_messages->send("me", $msg);
}
credentials.json refers to a service user