Prerequisites

• Install packages: Azure.Identity and Microsoft.Graph.
• Register an application in Azure Entra ID and grant Mail.Send permissions (Application or Delegated)

using Azure.Identity;
using Microsoft.Graph;
using Microsoft.Graph.Models;
using Microsoft.Graph.Users.Item.SendMail;

// Credentials
string tenantId = "YOUR_TENANT_ID";
string clientId = "YOUR_CLIENT_ID";
string clientSecret = "YOUR_CLIENT_SECRET";
string userId = "sender@yourdomain.com"; // User ID or User Principal Name

string filePath = "C:\\document.pdf";
byte[] fileBytes = await File.ReadAllBytesAsync(filePath);
string base64Content = Convert.ToBase64String(fileBytes);

var attachment = new FileAttachment
{
    OdataType = "#microsoft.graph.fileAttachment",
    Name = "document.pdf",
    ContentType = "application/pdf",
    ContentBytes = Convert.FromBase64String(base64Content)
};


var scopes = new[] { "https://graph.microsoft.com/.default" };

// Authenticate using Client Secret
var credential = new ClientSecretCredential(tenantId, clientId, clientSecret);
var graphClient = new GraphServiceClient(credential, scopes);

// Create the message
var requestBody = new SendMailPostRequestBody
{
    Message = new Message
    {
        Subject = "Hello from Microsoft Graph",
        Body = new ItemBody
        {
            ContentType = BodyType.Text,
            Content = "This is a test email sent using C# and Microsoft Graph."
        },
        ToRecipients = new List<Recipient>
        {
            new Recipient
            {
                EmailAddress = new EmailAddress
                {
                    Address = "recipient@example.com"
                }
            }
        },
        Attachments = new List<Attachment> { attachment }
    },
    SaveToSentItems = true
};

// Send the email
await graphClient.Users[userId].SendMail.PostAsync(requestBody);