假定邮箱确实存在！

可能是由于各种原因.您需要一一看待它们.
港口开放吗?防火墙权限到位了吗?
进一步确保已在Web.Config中配置SMTP配置:

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>

如果需要，请查看此Microsoft Video教程:
使用ASP.NET发送来自网站的电子邮件 [在ASP.NET中发送电子邮件的教程 [

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[^]