using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

public class Program

{

    public static void Main()

    {

        var emailswithIDs = new List<string>() {

            "1", "abc@gmail.com", 

            "2", "abcd@gmail.com", 

            "3", "abcde@gmail.com",

        };

      Dictionary<string, string> emailToId = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);

​

      for (int i = 0; i < emailswithIDs.Count; i += 2) 

        emailToId.Add(emailswithIDs[i + 1], emailswithIDs[i]);

      var emails = new List<string> {

          "abcd@gmail.com"

      };

      var ids = emails

        .Where(email => emailToId.ContainsKey(email))

        .Select(email => emailToId[email]);

        Console.WriteLine(string.Join(", ", ids));

    }

}