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 " ;