GMail not showing inline-images (cid) i'm sending with System.Net.Mail

The inline-image is ignored in GMail webinterface when added as attachment. When adding the image as alternate view it gets ignored by Outlook.

To add an inline-image compatible to GMail webinterface and Outlook (and iPhone mail client) you have to add it as LinkedResource.

The example code in the question must be fixed like this:

SmtpClient client = new SmtpClient("real.server.on.the.internet");
MailMessage mail = new MailMessage("Flattiverse <[email protected]>", "Ghostie <[email protected]>");
mail.BodyEncoding = System.Text.Encoding.UTF8;
mail.SubjectEncoding = System.Text.Encoding.UTF8;

LinkedResource image = new LinkedResource("test.png", "image/png");
image.ContentId = "[email protected]";
image.TransferEncoding = System.Net.Mime.TransferEncoding.Base64;
image.ContentType.Name = "[email protected]";
image.ContentLink = new Uri("cid:[email protected]");

AlternateView plainView = AlternateView.CreateAlternateViewFromString("Please view as HTML-Mail.", System.Text.Encoding.UTF8, "text/plain");
plainView.TransferEncoding = System.Net.Mime.TransferEncoding.QuotedPrintable;

AlternateView htmlView = AlternateView.CreateAlternateViewFromString("Image there?<br /><img src=\"cid:[email protected]\" /><br />Hope so!", System.Text.Encoding.UTF8, "text/html");
htmlView.LinkedResources.Add(image);
htmlView.TransferEncoding = System.Net.Mime.TransferEncoding.QuotedPrintable;
mail.AlternateViews.Add(plainView);
mail.AlternateViews.Add(htmlView);

mail.Subject = "15";

client.UseDefaultCredentials = false;
client.Credentials = new System.Net.NetworkCredential("working_username", "working_password");
client.Send(mail);

I had the same Problem with Pyhon (Django). Solved it by just adding the X-Attachment-Id header:

img.add_header('Content-ID', '<filename.png>')
img.add_header('X-Attachment-Id', 'filename.png')
img.add_header('Content-Disposition', 'inline', filename='filename.png')
message.attach(img)

Hope this helps someone :-)

I had the same issue (in Java, will be same for c#). Resolved by adding contentId between the “<” and “>”

This one is working on Gmail,yahoo and Outlook.