Cannot Get IIS Pickup Directory Error / How To Fix It

i i wrote an asp application which sends mail.following is the code:

MailMessage Mail = null;
SmtpClient Client = null;
MailAddress From;
Mail = new MailMessage();
Client = new SmtpClient();
From = new MailAddress(dhoni@domainName.com);
Mail.From = From;
Mail.Subject = Demo mail;
Mail.Body = hai this is sample mail;
Mail.To.Add(hemanth@domainName.com);
Client.Host = strHostName;
Client.DeliveryMethod = SmtpDeliveryMethod.PickupDirectoryFromIis;
Client.Send(Mail);

it works in windows xp operating ssyste.

when i run the same code in Windows vista and windows seven i am getting an error specifying "Cannot get IIS pickup directory".

i searched in net....

some suggests that use SmtpDeliveryMethod.Network;

why we are using this(Network) instead of PickupDirectoryFromIis...