Importing System.Net.Mail in Global.asax

Just add

using System.Net.Mail;

in your code file of your Global.asax.

In case if dont have a cs file , use a script tag for adding the references

<script runat="server">
using System.Net.Mail; 
</script>

If you want to add global.asax file can have a reference here .
http://www.aspdotnet-suresh.com/2011/05/how-to-add-globalasaxcs-file-in-aspnet.html[^]