I am using system.net.mail class to send mails from asp.net.
When i am sending mails the mails are sent although but the delivery is delayed. I am getting mails after 30 - 40 minutes.
Do you know what would be the reason,
  In this scenario you need to talk to your Network Administrator or ISP (Internet services provider) . If you are using Shared hosting / dedicated hosting
you need to open support ticket with them. If they fail to answer warn them; you will terminate their services. And look for another host.

Satalaj