DataTable loadDt = (DataTable)((BindingSource)deGridViewLoad.GridControl.DataSource).DataSource;
var loadMails = (from loadMail in loadDt.AsEnumerable()
                      where loadMail["AN8"].ToString().Trim() == 164488.ToString().Trim()  // поле, которое считывается с Excel
                      select loadMail["E-MAIL"].ToString().Trim());

           DataTable mainDt = (DataTable)((BindingSource)deGridViewMain.GridControl.DataSource).DataSource;
           var mainMails = (from mainMail in mainDt.AsEnumerable()
                            where mainMail.Field<int>("MLAN8") == 164488
                            select mainMail.Field<string>("MLEMAIL").ToString().Trim());
           string strMails = "";
           strMails = mainMails.Aggregate((a, b) => a + "; " + b);
           //strMails = string.Join("; ", Enumerable.ToArray<string>(mainMails));

           MessageBox.Show(strMails);