i am sending the html emai by using smtp servre

Make sure that the email address you're attempting to deliver email to is valid, or the mailbox is not full and is allowed to receive email.

Then, make sure you have configured SMTP configuration in Web.Config:

<system.net>
   <mailSettings>
     <smtp from="abc@somedomain.com">
       <network host="somesmtpserver" port="25" userName="name" password="pass" defaultCredentials="true" />
     </smtp>
   </mailSettings>
</system.net>

If needed, have a look at this Microsoft Video tutorial:
Use ASP.NET to send Email from Website[^]

Also, have a look at this discussion on similar issue: Mailbox unavailable error[^]

It says: "This happens when you specify a domain with your NetworkCredentials. If you specify a domain (third argument) then you can only send to valid mailboxes within that domain."

hii

refer this

Send Email in ASP.Net 2.0 - Feed back Form[^]