unable to send email via microsoft graph api with delegated permission

The code you provided shows you use client credential flow to do the authentication. When you use Mail.Send Application permission, use client credential flow is ok. But if you use Mail.Send Delegated permission, we can not use client credential. You should use username/password flow to do authentication.

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Below is my code:

using Microsoft.Graph;
using Microsoft.Graph.Auth;
using Microsoft.Identity.Client;
using System;
using System.Collections.Generic;
using System.Security;

namespace ConsoleApp34
{
    class Program
    {
        static async System.Threading.Tasks.Task Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            var publicClientApplication = PublicClientApplicationBuilder
            .Create("client id")
            .WithTenantId("tenant id")
            .Build();

            string[] scopes = new string[] { "mail.send" };

            UsernamePasswordProvider authProvider = new UsernamePasswordProvider(publicClientApplication, scopes);

            GraphServiceClient graphClient = new GraphServiceClient(authProvider);

            var message = new Message
            {
                Subject = "Meet for lunch?",
                Body = new ItemBody
                {
                    ContentType = BodyType.Text,
                    Content = "The new cafeteria is open."
                },
                ToRecipients = new List<Recipient>()
                {
                    new Recipient
                    {
                        EmailAddress = new EmailAddress
                        {
                            Address = "to email address"
                        }
                    }
                }
            };

            var securePassword = new SecureString();
            foreach (char c in "your password")
                securePassword.AppendChar(c);

            var saveToSentItems = true;

            await graphClient.Me
                    .SendMail(message, saveToSentItems)
                    .Request().WithUsernamePassword("your email", securePassword)
                    .PostAsync();
        }
    }
}

The reason for your error message Insufficient privileges to complete the operation is you use the code:

User me = await graphClient.Me.Request()
                .WithUsernamePassword("<username>", secureString)
                .GetAsync();

This code is used to get the user(me)’s information but not send email, you haven’t added the permission to the app. So it will show Insufficient privileges to complete the operation. Please remove this code and use the code block in my code instead:

await graphClient.Me.SendMail(message, saveToSentItems)
                    .Request().WithUsernamePassword("your email", securePassword)
                    .PostAsync();

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