Concurrent requests to send emails will generate duplicate emails, and the number does not match.

Hello！

I used Postman to iterate 160 times to send emails, and actually received 160+ emails. The number of emails received is random and does not match the number, which means duplicate emails are generated.

@PostMapping("/test") public void SysEmailController() { GraphEmailSender.testInfo(); }

`

import com.azure.identity.ClientSecretCredential;
import com.azure.identity.ClientSecretCredentialBuilder;
import com.microsoft.graph.models.EmailAddress;
import com.microsoft.graph.models.ItemBody;
import com.microsoft.graph.models.Message;
import com.microsoft.graph.models.Recipient;
import com.microsoft.graph.serviceclient.GraphServiceClient;
import com.microsoft.graph.users.item.sendmail.SendMailPostRequestBody;

import java.util.Collections;
import java.util.List;

public class GraphEmailSender {
// 配置参数
private static final String CLIENT_ID = "";
private static final String CLIENT_SECRET = "";
private static final String TENANT_ID = "";
private static final String SENDER_EMAIL = "";
private static final String[] SCOPES = new String[] { "https://graph.microsoft.com/.default" };

private static GraphServiceClient graphClient;


static {
    try {
        final ClientSecretCredential credential = new ClientSecretCredentialBuilder()
                .clientId(CLIENT_ID)
                .tenantId(TENANT_ID)
                .clientSecret(CLIENT_SECRET)
                .build();

        if (null == credential) {
            throw new Exception("Unexpected error");
        }

        graphClient = new GraphServiceClient(credential, SCOPES);
    } catch (Exception e) {
        System.err.println("init GraphServiceClient error: " + e.getMessage());
        e.printStackTrace();
    }
}

public static void testInfo() {
    try {

        Message message = new Message();
        message.setSubject("test mail - Microsoft Graph API");

        ItemBody body = new ItemBody();
        body.setContent("Microsoft Graph API mail。");
        message.setBody(body);

        Recipient recipient = new Recipient();
        EmailAddress emailAddress = new EmailAddress();
        emailAddress.setAddress("");
        recipient.setEmailAddress(emailAddress);
        List<Recipient> recipients = Collections.singletonList(recipient);
        message.setToRecipients(recipients);

        SendMailPostRequestBody sendMailPostRequestBody =
                new SendMailPostRequestBody();
        sendMailPostRequestBody.setMessage(message);

        //
        graphClient.users()
                .byUserId(SENDER_EMAIL)
                .sendMail()
                .post(sendMailPostRequestBody);

        System.out.println("send success！"+Thread.currentThread());

    } catch (Exception e) {
        System.err.println("error: " + e.getMessage());
        e.printStackTrace();
    }
}

}`

Start with the provided @PostMapping("/test") entry point and GraphEmailSender.testInfo(), then reproduce the 160 concurrent sendMail requests using the shown Microsoft Graph client code. Compare request and delivery counts to determine whether the behavior is in the SDK or the calling application. Done means a reproducible SDK defect with a focused test or clear evidence that no SDK change is indicated.