For sending mail, you can use
System.Net.Mail
Sample is given over
here[
^].
Now all you have to do is, get all the users from the database, store it in
DataTable
and then,
foreach(DataRow dr in dt.Rows)
{
SendMailToClient(dr["Email"].ToString());
}
Hope the flow is clear to you.
There is a another way for doing this, but once you got the idea, you'll have that as well, for sure :)
-KR