DotNetOpenMail.EmailMessage email = new DotNetOpenMail.EmailMessage(); email.FromAddress = new DotNetOpenMail.EmailAddress("zhifengwu1211@126.com"); DotNetOpenMail.EmailAddressCollection toAddresses = new DotNetOpenMail.EmailAddressCollection(); DotNetOpenMail.EmailAddress toAddress = new DotNetOpenMail.EmailAddress("zhifengwu1211@126.com"); toAddresses.Add(toAddress); email.ToAddresses.Add(toAddress); email.Subject = "this is test"; DotNetOpenMail.SmtpServer smtpserver = new DotNetOpenMail.SmtpServer("192.168.2.129"); StreamReader readfile = new StreamReader("d:\\mycert.cer"); Byte[] fileBytes = System.Text.Encoding.Default.GetBytes(readfile.ReadToEnd()); DotNetOpenMail.FileAttachment fileAttachment = new DotNetOpenMail.FileAttachment(fileBytes); fileAttachment.FileName = "mycert.cer"; email.AddMixedAttachment(fileAttachment); bool result=email.Send(smtpserver); if (result == true) this.label1.Text = "success"; else this.label1.Text = "error";