ASP.NET

//edited, as retrieving the string may not be apparent:
string emailString = NameOfTextBox.Text;
string[] emailArr = emailString.Split(';');

//remember "using System.Text"
StringBuilder sb = new StringBuilder();

foreach (string s in emailArr)
  sb.AppendFormat("{0};", s);

string rebuiltString = sb.ToString();

StringBuilder sb = new StringBuilder();

using (SqlConnection conn = new SqlConnection("(replace this with a real connection string)"))
{
  conn.Open();
  using (SqlCommand cmd = new SqlCommand("SELECT Email FROM Customers WHERE newsletterAgree = 1", conn))
  {
     using (SqlDataReader rdr = cmd.ExecuteReader())
     {
       while (rdr.Read())
         sb.AppendFormat("{0};", rdr["Email"]);
     }
  }
}

string finalString = sb.ToString();

string[] finalArr = finalString.Split(';');