Hi,

Reason because of might be the permission so please specify below path for pickup directory:

C:\inetpub\mailroot\Pickup

Code for PickupDirectoryFromIis:

Asp.net code :

  var msgMail = new MailMessage(from mail, tomail, subject, body);

                msgMail.IsBodyHtml = false;

                var server = new SmtpClient("localhost");

                server.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;

                senderMail.PickupDirectoryLocation = @"C:\inetpub\mailroot\Pickup";               

                server.UseDefaultCredentials = false;