how to add sender details while sending message 2 outlook

Question

how to add sender details(email id) while sending an email to receivers id having outlook id .here is my code

using lookout = Microsoft.Office.Interop.Outlook;

using outlookapp = Microsoft.Office.Interop.Outlook.Application;

using mapiNamespace = Microsoft.Office.Interop.Outlook.NameSpace;

using CalendarFolder = Microsoft.Office.Interop.Outlook.MAPIFolder;

using outlookCalendarItems = Microsoft.Office.Interop.Outlook.Items;

using outlookrecepient = Microsoft.Office.Interop.Outlook.Recipient;

using outlookrecepients = Microsoft.Office.Interop.Outlook.Recipients;

 outlookapp outlook = new outlookapp();

            MailItem mailItem = (MailItem)outlook.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);

            mailItem.To = oulookid;

            mailItem.Subject = "LEAVE GRANTED FROM " + startdate + "to" + enddate;

            mailItem.Body = "ENJOY THE HOLIDAYS";

            mailItem.Importance = OlImportance.olImportanceHigh;

           mailItem.Sender =  /* what to write*/          

            mailItem.Send();

thanks in advance