Why are there errors when sending email to a local server when the Internet is off?

Question

Laravel

Hello, tell me please. There is a feedback form on the site, I test sending messages on the local server, if the Internet is on, everything is OK, messages come to the mail. If the Internet is turned off, I get the following error:

[16:46:01] LOG.error: [INVALID UTF-8 DATA]
[16:46:01] LOG.error: Malformed UTF-8 characters, possibly incorrectly encoded {"exception":{}}

And it’s also strange that in my OpenServer (local server) it’s configured that messages should be saved to a folder, but when the Internet is on, messages are sent immediately to the mail, why they are immediately sent to the mail, I don’t quite understand.
Controller method that sends messages:

public function index(Request $request)
    {
        $validator = Validator::make($request->all(), [
            'name' => 'required',
            'phone' => 'required|regex:/^\+?[0-9 \-()]+$/i',
            'email' => 'regex:/\.*[a-z0-9A-Z][email protected]{1}[a-z0-9A-Z]+\.*/i'
        ]);
        if($validator->fails()) {
            $errors = $validator->errors();
            return response()->json([
                'status' => false,
                'body' => array_keys($errors->messages()),
            ]);
        }
        Mail::send(new QuestionShipped($request->all()));
        $msg = new letter();
        $msg->store($request);
        return response()->json([
            'status' => true,
        ]);
    }

Please tell me what could be the problem, thanks in advance for the answer.