Let the data table variable be inputDT
Use assign activity
On the Left side use a new a variable say MailString of type String
On the right side use the code below
String.Join(Environment.NewLine, inputDT.AsEnumerable.Select(Function (x) String.Join(" ", {"Dear", x(0).ToString, x(1).ToString, x(2).ToString})))
you will get the required string stored in the String variable MailString. You can use this string variable in mail Body