You can take bind the users inside one dropdown and get email on the textBox when value changes from ddList
for ex:
First set the AutoPostBack=True for all the dropdownlist. then implement the selectedIndexChanged This way
Bind dropdown with usernames and on Implement its selectgedIndexChange
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
string s = DropDownList1.SelectedValue.ToString();
SqlConnection conn = new SqlConnection();
conn.ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
SqlDataAdapter da = new SqlDataAdapter("select email from table1 where user='" + s + "'", conn);
DataTable dt = new DataTable();
da.Fill(dt); // Fill the dataset
txtEmail.Text = dt.Rows[0][0];