Mailbox unavailable error found

Assuming the mailbox does exists!

It can be because of various reasons. You need to look at them one by one.
Is the port open? Firewall permissions in place?
Further 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[^]
Tutorials on sending Email in ASP.NET[^]