当接收方的电子邮件地址不正确时，Smtp.Send不会引发任何错误

Question

I am sending email using SmtpClient.Send Method , but when I try to send to a non-existent email account, the Send method throws no exception, how can I tell that the recipient mail address is valid, or the mail won't arrive, or detect this issue.

code:

            Properties.Settings appSettings = new Properties.Settings();
            MailMessage message = new MailMessage();

            message.From = new MailAddress(appSettings.senderMail);        
            message.Subject = appSettings.mailsubj;
            message.Body = appSettings.mailbody;
            Attachment attach = new Attachment(sOutput);
            message.Attachments.Add(attach);

            SmtpClient client = new SmtpClient(appSettings.SMTP);

            client.SendCompleted += this.SendCompletedCallback;
            foreach (String clientEmail in clientEmailList)
            {
                message.To.Clear();
                message.To.Add(clientEmail);
                //client.Send(message);

                try
                {
                    client.Send(message);
                    AddToLog(String.Format("\t{0}{1}", Properties.Resources.ResourceManager.GetString("MAIL_SEND_SUCCESS"), clientEmail));
                }
                catch (SmtpFailedRecipientException ex)
                {
                    AddToLog(String.Format("\t{0}{1}:\n\t{2}", Properties.Resources.ResourceManager.GetString("MAIL_SEND_ERROR"), clientEmail, ex.Message));
                }
                catch (SmtpException smtp_Ex)
                {
                    AddToLog(String.Format("\t{0}{1}:\t{2}", Properties.Resources.ResourceManager.GetString("MAIL_SEND_ERROR"), clientEmail, "Cannot connect to SMTP server."));
                }
                catch (Exception ex)
                {
                    AddToLog(String.Format("\t{0}{1}:\n\t{2}", Properties.Resources.ResourceManager.GetString("MAIL_SEND_ERROR"), clientEmail, ex.Message));
                }
              //  client.SendAsync(message, clientEmail);
            }

Answer 1

When SMTP client sends a message, it doesn't know anything about validity of e-mail address, if this address belongs to the domain, that differs from SMTP server's domain.

This will be known only after your SMTP server will try to deliver a message to the recipient from another domain. Usually, in this case you'll receive a mail message with error.