You need to create a new row each time - and some columns would help as well!
Try this:
char[] delimiterChars = { ' ', ',', '.', ':', '\t' };
string Text = exDT.Rows[i]["Sender Name"].ToString();
string[] words = Text.Split(delimiterChars);
DataTable dt1 = new DataTable();
dt1.Columns.Add("Word", typeof(string));
foreach (string s in words)
{
dt1.Rows.Add(s);
}