MailMessage myMail = new MailMessage();
SmtpClient mySmtpServer = new SmtpClient("smtp.gmail.com");
myMail.From = new MailAddress("abc@abc.org", "Test Group");
myMail.To.Add("abc@abc.org");
myMail.Bcc.Add("abc@abc.org");
myMail.Subject = "Test";
string myFile3 = pdfPath;
Attachment myAttach3 = new Attachment(myFile3, MediaTypeNames.Application.Octet);
ContentDisposition disposition3 = myAttach3.ContentDisposition;
disposition3.FileName = "ticket.pdf"; <

4.5 以上， 4.0以下太长常会出问题

http://goo.gl/092ypf try it