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

Question

When I send an email via outlook or gmail to a gmail email address I can add inline-images which are directly shown in the gmail webinterface:

Relevant raw mail-header and raw body parts of the working email:

--089e0158b6909948880520cef593
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr">Image there?<div><img src=3D"cid:ii_if3zqhar0_15014363be0a=
41b2" width=3D"10" height=3D"3"><br>=E2=80=8BHope so!<br></div></div>

--089e0158b6909948880520cef593--
--089e0158b69099488c0520cef594
Content-Type: image/png; name="test.png"
Content-Disposition: inline; filename="test.png"
Content-Transfer-Encoding: base64
Content-ID: <ii_if3zqhar0_15014363be0a41b2>
X-Attachment-Id: ii_if3zqhar0_15014363be0a41b2

iVBORw0KGgoAAAANSUhEUgAAAAoAAAADCAIAAAAlXwkiAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJ
bWFnZVJlYWR5ccllPAAAADFJREFUeNpi+A8BDCf/wwDD/1VIbBABIudDmAchokwgag9QAiwHVcsM
Z/5fCdYJEGAAuthJ+AVi5KgAAAAASUVORK5CYII=
--089e0158b69099488c0520cef594--

Full working raw email: Working raw-email.

However, when I send such an email via System.Net.Mail from .NET it is not working in the gmail webinterface but any other email client (outlook, iphone, etc.):

Relevant raw mail-header and raw-body parts of non-working email:

----boundary_3_6a0761ee-57e2-4bdd-b1f1-7302b3c8a7a1
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Image there?<br /><img src=3D"cid:[email protected]" /><=
br />Hope so!
----boundary_3_6a0761ee-57e2-4bdd-b1f1-7302b3c8a7a1--

----boundary_5_979e00c0-3fb9-46a0-b25c-1cee82cc15eb
Content-Type: image/png; name=test.png
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename=test.png
Content-ID: <[email protected]>

iVBORw0KGgoAAAANSUhEUgAAAAoAAAADCAIAAAAlXwkiAAAAGXRFWHRTb2Z0d2FyZQBB
ZG9iZSBJbWFnZVJlYWR5ccllPAAAADFJREFUeNpi+A8BDCf/wwDD/1VIbBABIudDmAch
okwgag9QAiwHVcsMZ/5fCdYJEGAAuthJ+AVi5KgAAAAASUVORK5CYII=
----boundary_5_979e00c0-3fb9-46a0-b25c-1cee82cc15eb--

Full non-working raw email: Nonworking raw-email.

This is my code to send inline-images:

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;

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.TransferEncoding = System.Net.Mime.TransferEncoding.QuotedPrintable;
mail.AlternateViews.Add(plainView);
mail.AlternateViews.Add(htmlView);

mail.Subject = "7";

Attachment attachment = new Attachment("test.png", "image/png");
attachment.ContentId = "[email protected]";
attachment.ContentDisposition.Inline = true;
attachment.ContentDisposition.DispositionType = "inline; filename=test.png";
mail.Attachments.Add(attachment);

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

I also tried cid in GMail format (eg. ii_012345678_9abcdef0123456789) and many other things stated in other related questions. (Using ' instead of " in mail body, etc.)

Question: What am I doing wrong that GMail doesn't display my inline-images? How do I need to change my code? Maybe what I want can't be achieved with System.Net.Mail?

Answer 1

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);

Answer 2

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 :-)