Sending Email - Urgent Help Needed

Dear experts,

I have written an application which will send out email notifications to my users.

I noticed that sometimes when i trigger a event which will sends out email, my application will load for a very long time to complete but sometimes, it will send almost immediately.

My exchange server is seperate server from my web server, but both of them is within the same network.

I tried to add asynchronous mailing but the random long loading time still persist.

I have also thought about putting the mail into a pickup directory so my web application will not load that long, but i am not sure how to configure it.

Can anyone please kindly advice? Please help....

Thanks.

public static void sendMail(List<User> users, string subject, string body)
        {
            try
            {
                MailMessage message = new MailMessage();
                message.From = new MailAddress(from);

                for (int i = 0; i < users.Count(); i++)
                {
                    message.To.Add(users[i].Email);
                }

                message.Subject = subject;
                message.Body = body;
                message.IsBodyHtml = true;

                SmtpClient emailClient = new SmtpClient();
                emailClient.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;

                 
                Object userState = message;

                try
                {
                    //Send the email asynchronously
                    emailClient.SendAsync(message, userState);
                }
                catch (SmtpException smtpEx)
                {
                    MailMessage errorMailAlert = new MailMessage();
                    errorMailAlert.From = new MailAddress(from);
                    errorMailAlert.To.Add(new MailAddress("randomise@scri.edu.sg"));

                    errorMailAlert.Subject = "Send Mail Failure";
                    errorMailAlert.Body = "system has failed to a mail. Details of the error is stated below and can be viewed" +
                        " in the MailLog under the rand log folder.<br/><br/>Exception details:<br/>" + smtpEx.ToString();
                    errorMailAlert.IsBodyHtml = true;

                    //try sending message to the admin
                    try
                    {
                        //Send the email asynchronously
                        emailClient.SendAsync(errorMailAlert, userState);
                    }
                    catch (SmtpException smtpEx2)
                    {
                        //Error handling here
                        ExceptionUtility.LogMailException(smtpEx2);
                    }
                    //Error handling here
                    ExceptionUtility.LogMailException(smtpEx);

                }
                catch (Exception ex)
                {
                    //Error handling here
                    ExceptionUtility.LogMailException(ex);
                }
            }
            catch
            {
                throw;
            }
        }

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