Provide a link:

<link href="../../Content/themes/base/jquery-ui.css" rel="stylesheet" type="text/css" />

write a script as:

$(function () {
              function split(val) {
                  return val.split(/,\s*/);
              }
              function extractLast(term) {
                  return split(term).pop();
              }
 
              $("#NewAddress").bind("keydown", function (event) {
                  if (event.keyCode === $.ui.keyCode.TAB &&
                      $(this).data("autocomplete").menu.active) {
                      event.preventDefault();
                  }
              })
              $("#NewAddress").autocomplete({
                  source: function (request, response) {
                      //define a function to call your Action (assuming UserController)
                      $.ajax({
                          url: '/Controller/action', type: "GET", dataType: "json",
                          //query will be the param used by your action method
                          data: { query: request.term },
                          term: extractLast(request.term),
                          success: function (data) {
                              response($.map(data, function (item) {
                                  return { label: item, value: item };
                              }))
                          }
                      })
                  },
                  search: function () {
                      // custom minLength
                      var term = extractLast(this.value);
                      if (term.length < 1) {
                          return false;
                      }
                  },
                  focus: function () {
                      // prevent value inserted on focus
                      return false;
                  },
                  select: function (event, ui) {
                      var terms = split(this.value);
                      // remove the current input
                      terms.pop();
                      // add the selected item
                      terms.push(ui.item.value);
                      // add placeholder to get the comma-and-space at the end
                      terms.push("");
                      this.value = terms.join(", ");
                      return false;
                  }
              });
 
          });

In the view body write:

<%=Html.TextBoxFor(m => m.NewAddress)%>

in the controller :

public ActionResult GetEmailIds(string query)
       {
           query = query.Replace(" ", "");
           if (query.Length > 1)
           {
               int op = query.LastIndexOf(",");
               query = query.Substring(op + 1);
           }
           var users = (from u in _userService.GetAllUsers()
                        where u.EmailAddress.Contains(query)
                        orderby u.EmailAddress // optional
                        select u.EmailAddress).Distinct().ToArray();
 
           return Json(users, JsonRequestBehavior.AllowGet);
       }

Autocomplete in ASP.NET MVC3