Send Mails unattended with Microsoft Graph API

I want to send mails unattended in a .NET console application with the Microsoft Graph API.

The API supports sending a mail directly or create a draft, modify it and send it. The first has a limitation on attachment size. Each attachment has to be under 3 MB in size. See Documentation

The latter one requires the Mail.ReadWrite API permission. Adding it as an application permission would enable the application to access every mailbox in the organisation, which I do not want to grant if possible.

Another option would be to use the username and password authentication flow. But our corporation AD requires multi-factor authentication, a constraint of that flow.

I tried to authentiate the application with the client credential flow. Sending an attachment over 3 MB requires the Mail.ReadWrite privilege.

Authenticating the application with my user account using the username/password did also fail, because of the MFA requirement.

I am using the NuGet packages Azure.Identity for the authentication flow and Microsoft.Graph to send the requests.

What is the proper way to send mails unattended with attachments over 3 MB from a .NET console application? Where did I take the wrong turn?

Thanks in advance and let me know if you need more details.

The client credentials flow is generally the best option as this avoids having a username and password which has a lot of associated problems (MFA is just one). With the issue of having full access to the tenant you can scope down the access https://learn.microsoft.com/en-us/graph/auth-limit-mailbox-access so your app can then only Send/Access particular mailboxes.

For sending emails over 3mb see https://learn.microsoft.com/en-us/graph/outlook-large-attachments?tabs=csharp if you use the Graph SDK it makes this relatively easy eg

        var fileStream = System.IO.File.OpenRead(bigAttachment.FullName);
        var uploadRequestBody = new Microsoft.Graph.Users.Item.Messages.Item.Attachments.CreateUploadSession.CreateUploadSessionPostRequestBody
        {
            AttachmentItem = new AttachmentItem
            {
                AttachmentType = AttachmentType.File,
                Name = bigAttachment.Name,
                Size = fileStream.Length,
                ContentType = "application/octet-stream"
            }
        };

        var uploadSession = graphClient.Users[objectID]
            .Messages[savedDraft.Id]
            .Attachments
            .CreateUploadSession
            .PostAsync(uploadRequestBody).GetAwaiter().GetResult();

        // Max slice size must be a multiple of 320 KiB
        int maxSliceSize = 320 * 1024;
        var fileUploadTask = new LargeFileUploadTask<FileAttachment>(uploadSession, fileStream, maxSliceSize);

        var totalLength = fileStream.Length;
        // Create a callback that is invoked after each slice is uploaded
        IProgress<long> progress = new Progress<long>(prog =>
        {
            Console.WriteLine($"Uploaded {prog} bytes of {totalLength} bytes");
        });
        try
        {
            // Upload the file
            var uploadResult = fileUploadTask.UploadAsync(progress).GetAwaiter().GetResult();
            Console.WriteLine(uploadResult.UploadSucceeded ? "Upload complete" : "Upload failed");
        }
        catch (ServiceException ex)
        {
            Console.WriteLine($"Error uploading: {ex.ToString()}");
        }