当我通过 Outlook 或 gmail 向 gmail 电子邮件地址发送电子邮件时，我可以添加直接显示在 gmail 网络界面中的内联图像：
工作电子邮件的相关原始邮件标头和原始正文部分：
--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--
完整的工作原始电子邮件：工作原始电子邮件.
但是，当我通过发送这样的电子邮件时System.Net.Mail from .NET它不适用于 gmail 网络界面，但适用于任何其他电子邮件客户端（outlook、iphone 等）：
非工作电子邮件的相关原始邮件标头和原始正文部分：
----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--
完整的非工作原始电子邮件：非工作原始电子邮件.
这是我发送内联图像的代码：
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);
我还尝试了 GMail 格式的 cid（例如ii_012345678_9abcdef0123456789）以及其他相关问题中所述的许多其他内容。 （在邮件正文等中使用 ' 代替 "）
问题：我做错了什么，GMail 不显示我的内嵌图像？我需要如何更改我的代码？也许我想要的无法实现System.Net.Mail?
添加为附件时，内联图像在 GMail Web 界面中会被忽略。将图像添加为备用视图时，Outlook 会忽略该图像。
要添加与 GMail Web 界面和 Outlook（以及 iPhone 邮件客户端）兼容的内嵌图像，您必须将其添加为LinkedResource.
问题中的示例代码必须像这样修复：
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);