How to send MIME email with attachments from Graph SDK? Preventing Graph API from Creating winmail.dat attachment

Question

I'm using graph sdk from c# to send emails from a web app. The body is formatted as HTML. I'm attaching PDFs to the email. The recipients are receiving the email with winmail.dat and sometimes noname. It seems like the email is coming in as plain text. Gmail seems to able to unpack winmail.dat, but not all clients seem to do this.

Some research tells me to use MIME format, but having trouble finding a concise example.

How can I send emails from graph api / sdk formatted as HTML with MIME with attachments without creating winmail.dat

Here's a snip from my create message code:

        private MimeMessage GetMimeMessage(string fromEmail, string subject, string htmlBody, IEnumerable<string> toRecipients, IEnumerable<string> ccRecipients, IEnumerable<string> attachments)
        {
            var message = new MimeMessage();

            foreach (var recipient in toRecipients)
                message.To.Add(MailboxAddress.Parse(recipient));
            
            foreach (var recipient in ccRecipients)
                message.Cc.Add(MailboxAddress.Parse(recipient));

            message.From.Add(MailboxAddress.Parse(fromEmail));
            message.Subject = subject;
            var builder = new BodyBuilder();
            builder.TextBody = htmlBody;
            builder.HtmlBody = htmlBody;
            
            foreach (var attachment in attachments)
                builder.Attachments.Add(attachment);
            
            message.Body = builder.ToMessageBody();
            return message;


        }

Send Code:

        private async Task SendMimeMessageWithGraph(string sendFromEmail, MimeMessage mimeMessage)
        {
            var stream = new MemoryStream();
            mimeMessage.WriteTo(stream);
            
            stream.Position = 0;
            StringContent MessagePost = new StringContent(Convert.ToBase64String(stream.ToArray()), Encoding.UTF8, "text/plain");

            var message = new Message();
            var mypost = await MessagePost.ReadAsStringAsync();
            var sendMailRequest = _GraphClient.Users[sendFromEmail].SendMail(message, true).Request().GetHttpRequestMessage();
            sendMailRequest.Content = MessagePost;
            sendMailRequest.Method = HttpMethod.Post;
            var sendResult = await _GraphClient.HttpProvider.SendAsync(sendMailRequest);
            Debug.WriteLine(sendResult.StatusCode);
        }

From: fromemail@fromemail.com
Date: Tue, 06 Sep 2022 21:39:47 -0400
Subject: Invoice #0000003
Message-Id: <2FWZXJIFTHU4.JPGM32IG50SI2@person1234>
To: Newperson@new.com
Cc: cc@cc.cc
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="=-9G8gCZS3S+XQhQ+1kLqUog=="

--=-9G8gCZS3S+XQhQ+1kLqUog==
Content-Type: text/plain; charset=utf-8

Please see attached invoice.<br><br>Thank you for your business,<br>Test<br>Person<br>222-999-1111<br>email@email.com<br><img height="50px" width="auto" src="data:image/svg+xml;base64,PD94b...>">
--=-9G8gCZS3S+XQhQ+1kLqUog==
Content-Type: text/html; charset=utf-8

Please see attached invoice.<br><br>Thank you for your business,<br>Test<br>Person<br>222-999-1111<br>email@email.com<br><img height="50px" width="auto" src="data:image/svg+xml;base64,PD94...">
--=-9G8gCZS3S+XQhQ+1kLqUog==--

I've followed this blog post: https://gsexdev.blogspot.com/2021/08/sending-mimemessage-via-microsoft-graph.html

It partially works; emails received by clients without o365/outlook do not render the html email and include winmail.dat attachment. So, I still have the same issue.

Answer 1

In my case, I needed to disable TNEF formatting by disabling RTF. This is done through the online Exchange Admin Center:

1. Mail Flow
2. Remote domains
3. Choose 'Default'(or whatever you have set)
4. Edit text and character set
5. List item
6. Use rich-text format: Never.
7. Save.

Now both Graph SDK Messages and MIME messages send emails via graph as expected!