C# - Optimize Large Array Of User Reminders

i have a trigger that executes asp.net page with method that first retrieves from DB users with needed details and then emails them with information they need like ie. users that does not have an image in their profile. Once i get data from DB i build large string with encoded HTML and assign it to MailMessage.Body tag. There are about 20000 users per action/night who get the mail. Stored procedures just get users based on criteria i need nothing to optimize there (after proper indexes been made). How do i optimize it? Can i create sort of queue in ASP.NET that it will be executed all day long without one time action? Maybe i can use async actions to send few emails per time.

protected void Page_Load(object sender, EventArgs e)
{
string type = Request.QueryString["type"];
string spName = "";
string message = "";
switch (type)
{
//getUsersWithoutPictures
case "uwp":
spName = "getUsersWithoutPictures";
message = getUsersWithoutPicturesTemplate();
break;
//getUsersWithoutText
case "uwt":
spName = "getUsersWithoutText";
message = getUsersWithoutTextTemplate();
break;
//getUsersWithoutEnteringWebsiteForTwoWeeks
case "uwewftw":......................................