从C＃发送带有附件的电子邮件，附件作为Thunderbird中的第1.2部分到达

显式填充ContentDisposition字段可以达到目的。

if (attachmentFilename != null){    Attachment attachment = new Attachment(attachmentFilename, MediaTypeNames.Application.Octet);    ContentDisposition disposition = attachment.ContentDisposition;    disposition.CreationDate = File.GetCreationTime(attachmentFilename);    disposition.ModificationDate = File.GetLastWriteTime(attachmentFilename);    disposition.ReadDate = File.GetLastAccessTime(attachmentFilename);    disposition.FileName = Path.GetFileName(attachmentFilename);    disposition.Size = new FileInfo(attachmentFilename).Length;    disposition.DispositionType = DispositionTypeNames.Attachment;    message.Attachments.Add(attachment);     }

顺便说一句 ，在使用Gmail的情况下，您可能对ssl安全甚至端口有一些例外！

smtpClient.EnableSsl = true;smtpClient.Port = 587;