The specific situation is that the code prompts a 202 status code after authentication, but the recipient has not received any email.

Using Microsoft Center’s tracking email, but unable to understand what failed
trace log

Microsoft Azure

Below is the code

public class SendEmail {

private static final String CLIENT_ID = "**";
private static final String CLIENT_SECRET = "**";
private static final String TENANT_ID = "****";
private static final String AUTHORITY = "https://login.microsoftonline.com/" + TENANT_ID;
private static final String GRAPH_API_URL = "https://graph.microsoft.com/v1.0/users/****/sendMail";

public static void main(String[] args) {
    try {
        ConfidentialClientApplication app = ConfidentialClientApplication.builder(
                        CLIENT_ID,
                        ClientCredentialFactory.createFromSecret(CLIENT_SECRET))
                .authority(AUTHORITY)
                .build();

        Set<String> scopes = new HashSet<>(Collections.singletonList("https://graph.microsoft.com/.default"));
        ClientCredentialParameters clientCredentialParam = ClientCredentialParameters.builder(scopes)
                .build();

        CompletableFuture<IAuthenticationResult> future = app.acquireToken(clientCredentialParam);
        IAuthenticationResult result = future.join();

        if (result != null) {
            sendEmail(result.accessToken());
        } else {
            System.out.println("Failed to acquire access token.");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
}

private static void sendEmail(String accessToken) throws Exception {
    String toEmail = "*****";
    String subject = "Test Email";
    JSONObject bodyContent = new JSONObject().put("content", "Hello, this is a test email sent from Java using Microsoft Graph API.")
            .put("contentType", "Text");
    JSONObject messageBody = new JSONObject().put("body", bodyContent);
    JSONObject toRecipient = new JSONObject().put("emailAddress", new JSONObject().put("address", toEmail));
    JSONObject message = new JSONObject().put("message", new JSONObject()
            .put("subject", subject)
            .put("body", bodyContent)
            .put("toRecipients", new JSONArray().put(toRecipient)));

    URL url = new URL(GRAPH_API_URL);
    HttpsURLConnection con = (HttpsURLConnection) url.openConnection();
    con.setRequestMethod("POST");
    con.setRequestProperty("Authorization", "Bearer " + accessToken);
    con.setRequestProperty("Content-Type", "application/json");
    con.setDoOutput(true);

    try (OutputStream os = con.getOutputStream()) {
        byte[] input = message.toString().getBytes("utf-8");
        os.write(input, 0, input.length);
    }

    int responseCode = con.getResponseCode();
    System.err.println( responseCode);
    BufferedReader in;
    if (responseCode == 200) {
        in = new BufferedReader(new InputStreamReader(con.getInputStream()));
    } else {
        in = new BufferedReader(new InputStreamReader(con.getErrorStream()));
        System.err.println( responseCode);
    }

    String inputLine;
    StringBuilder response = new StringBuilder();

    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    in.close();

    if (responseCode == 200) {
        Map<String, String> map = JSON.parseObject(response.toString(), HashMap.class);
    }
    System.err.println(response.toString());

}

}

Thank you very much for helping me answer this question!!!