Check if UserExist or not while Adding new user by Email ID

I want to check whether the user with the Email ID exist or not.
Currently I am adding like this only. I want to check by EmailID. 
Here is my code
protected void btnSubmit_Click(object sender, EventArgs e) { if (Request.QueryString.Count > 0) { string StrPwd = Encrypt(txtPassword.Text.Trim()); using (SqlConnection conn = new SqlConnection(conString)) if (Request.QueryString["Id"] != null) { string Id = Request.QueryString["Id"]; SqlCommand cmd = new SqlCommand(); cmd.CommandText = " Update tbl_User SET username=@username,email=@email,active=@active,usertype=@usertype,ngoname=@ngoname,teachers_modules=@teachers_modules,student_modules=@student_modules,staff_modules=@staff_modules, patient_modu ...