邮件内容调用图片格式为：<img src=\"cid:Email001\">

发送邮件的服务端代码为：

SmtpClient 发送邮件的对象 //代码省略

System.Net.Mail.MailMessage mailMessage = new System.Net.Mail.MailMessage();
mailMessage.From="发送者邮箱";
mailMessage.To.Add("收件人邮件列表");
mailMessage.CC.Add("抄送人邮件列表");
mailMessage.Subject = subject;
AlternateView htmlBody = AlternateView.CreateAlternateViewFromString(content,null,"text/html");
LinkedResource lrImage = new LinkedResource("a.jpg","image/gif");
lrImage.ContentId = "Email001";
htmlBody.LinkedResources.Add(lrImage);
mailMessage.AlternateViews.Add(htmlBody);
SmtpClient.Send(mailMessage);